Excimer laser inspection system

ABSTRACT

A system and method for inspecting a specimen, such as a semiconductor wafer, including illuminating at least a portion of the specimen using an excimer source using at least one relatively intense wavelength from the source, detecting radiation received from the illuminated portion of the specimen, analyzing the detected radiation for potential defects present in the specimen portion.

This application is a continuation in part of U.S. patent applicationSer. No. 09/796,117, filed Feb. 28, 2001, now U.S. Pat. No. 6,842,298entitled “Broad Band DUV/VUV Long Working Distance Catadioptric ImagingSystem,” inventors Shafer et. al., which claims the benefit of U.S.Provisional Patent Application 60/231,761, filed Sep. 12, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of optical imagingand more particularly to optical systems for microscopic imaging,inspection and/or lithography applications.

2. Description of the Related Art

Many optical systems and electronic systems are available to inspectsurface features of a specimen for defects, including specimens such asa semiconductor photomask or partially fabricated integrated circuit.Defects on such specimens may take the form of particles randomlylocalized on the specimen surface, scratches, process variations,repeating pattern defects, and so forth. Techniques and devices forinspecting specimens for these microscopic defects are generallyavailable in the art and are embodied in various commercially availableproducts, including those available from KLA-Tencor Corporation of SanJose, Calif., the assignee of the present application.

The aim of virtually any type of inspection system or technique is torapidly and efficiently detect defects. With smaller and smallerfeatures on specimen surfaces and the use of new materials and newmanufacturing processes, detection of new and finer defects is required.It is also preferable to rapidly inspect a specimen surface in as shortan amount of time as possible, from loading the specimen to removing itfrom the inspection position and characterizing the defects. Such speedrequirements in the presence of smaller features mandates continuousimprovements in the available techniques to accurately and adequatelyfind specimen problems.

Inspection systems are available for wafer inspection, while stillothers target photomask inspection. The inspection systems currentlyavailable are highly complex, requiring a sophisticated combination oflight source, illumination, imaging, positioning, automatic focusing,image sensor, data acquisition, and data analysis subsystems. A completechange in the inspection system may be required to allow the detectionof new and smaller defects on a specimen.

Of the currently available systems, it should be noted that most use anon-pulsed light source from an arc lamp or a laser. A non-pulsed lampor laser provides a relatively constant power to the specimen and ismore easily implemented in a high speed inspection system. However,relatively constant and non-pulsed energy sources suffer from particulardrawbacks. Short wavelengths have desirable characteristics forinspecting small defects. Few non-pulsed sources are available with therequired power and brightness for high speed inspection at wavelengthsbelow 300 nm. In addition, non-pulsed laser light sources do not producelight energy with relatively low spatial and temporal coherence, whichcan be a problem in certain circumstances. For this reason, thesenon-pulsed laser sources suffer from interference and speckle inducedillumination discontinuities. Overcoming these problems requires timeaveraging of speckle patterns, such as by using a rotating ground glassplate.

It would therefore be desirable to have a system for inspecting aspecimen that improves upon the systems previously available, and inparticular for enabling inspection of specimens such as wafers andphotomasks with short wavelength light that do not have the adverseeffects associated with non-pulsed light sources.

SUMMARY OF THE INVENTION

The present invention is a system and method for inspecting a specimen,such as a semiconductor wafer or photomask, including illuminating atleast a portion of the specimen using an excimer source employing atleast one relatively intense wavelength from said source, detectingradiation received from the illuminated portion of the specimen, andanalyzing the detected radiation to view potential defects present inthe portion of the specimen.

These and other aspects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates functional aspects of the system disclosed herein;

FIG. 2 illustrates an aspect of the design used to reduce the peak powerof a laser pulse, and one that can be altered by varying the angles ofthe components to reduce speckle contrast for a single laser pulse;

FIG. 3 shows a plot of the intensity of a single pulse;

FIG. 4 is a plot of the intensity of multiple pulses, specifically eightpulses, resulting from the use of the system and method similar to theone illustrated in FIG. 2;

FIG. 5 is a delay arrangement using two prisms, each prism including aTIR surface and an AR surface;

FIG. 6 illustrates two prisms rotated such that light energy enteringmakes a total of six round trips between prisms, thereby increasingoverall delay time;

FIG. 7 is a delay arrangement employing a single image relay lens;

FIG. 8 illustrates a delay arrangement having two image relay lenses;

FIG. 9 presents a delay arrangement wherein three prisms are used eachhaving a TIR surface and incorporating a Brewster's angle surface;

FIG. 10 is a preferred angular arrangement of pulses to apply to groundglass to reduce speckle contrast in accordance with the design of FIG.2;

FIG. 11 shows the results of a standard laser pulse, the use of two DUVpulses, and four DUV pulses and the associated speckle contrasts;

FIG. 12 is an alternate aspect of the method and apparatus for reducingspeckle contrast employing an angularly offset diffraction grating;

FIG. 13 is the resultant speckle contrast of the grating arrangementused in FIG. 12;

FIG. 14 illustrates a functional diagram of the elements used in adevice that reduces peak power and speckle contrast;

FIG. 15 is a schematic side view of a zooming catadioptric imagingsystem in an application for the inspection of specimens includingsemiconductor wafers;

FIG. 16 illustrates a method of optical imaging and bright field anddark field inspection at wavelengths at or below 365 nm;

FIG. 17 is a multimode catadioptric dark field imaging systemconstructed to have a 0.97 NA;

FIG. 18 is a cutaway side view of dome-shaped reflector and the fusedsilica lens mirror element;

FIG. 19 illustrates the dimensions of the catadioptric group in oneaspect;

FIG. 20 presents a catadioptric dark field imaging system and having avariable angle of incidence from an illumination source;

FIG. 21 is a pupil aperture that may be used with the catadioptricsystem when selecting the directional dark field mode;

FIG. 22 presents an aperture that may be used with the catadioptricsystem when selecting the double dark field mode;

FIG. 23 illustrates operation in the central dark ground mode usingnormal illumination;

FIG. 24 illustrates an aperture that may be used with the catadioptricsystem when selecting the Manhattan geometry mode;

FIG. 25 illustrates a side view of a double dark field design systemthat illuminates the object at a relatively high angles of incidence;

FIG. 26 is a top view of the double dark field system;

FIG. 27 presents a central dark ground imaging system wherein the laserpasses through the collector at an approximately perpendicular angle tothe object;

FIG. 28 is a top view of the normal dark field imaging system;

FIG. 29 is a multimode catadioptric dark field imaging systemconstructed to have a 0.98 NA;

FIG. 30 is a multimode catadioptric dark field imaging systemconstructed to have a 0.99 NA;

FIG. 31 is a system having increased field size, specificallyapproximately 4.0 millimeters;

FIG. 32 presents a catadioptric system having a relayed pupil plane;

FIG. 33 presents an additional catadioptric system with a relayed pupilplane;

FIG. 34 illustrates the simultaneous operation of double dark field anddirectional dark field modes in the system disclosed herein;

FIG. 35 shows an expanded view of the tube lens group;

FIG. 36 presents a tube lens group for use with varying NA systems andproviding mapping of a 1.0 millimeter field size image onto a 36millimeter detector;

FIG. 37 is a six element varifocal tube lens group for providingmagnifications from 20× to 200×;

FIG. 38 illustrates the four elements of the varifocal tube lens groupfurthest from the pupil plane;

FIG. 39 is a schematic side view of a catadioptric imaging system inaccord with the parent application;

FIG. 40 is a schematic side view of a catadioptric imaging system;

FIG. 41 is schematic side view of a catadioptric imaging system in thethree positions having 36×, 64× and 100× power magnifications;

FIG. 42 is a schematic side view of a catadioptric imaging system inthree positions having 36×, 64× and 100× power magnifications;

FIG. 43 is a schematic side view of a catadioptric imaging system inthree positions having 36×, 64× and 100× power magnification;

FIG. 44 is a prior art system that completely corrects for primary,secondary and tertiary axial color over a broad wavelength band in thenear and deep ultraviolet (0.2 micron to 0.4 micron), but not forresidual lateral color;

FIG. 45 is a modified version of the '976 Shafer patent optimized foruse in 0.193 micron wavelength high power excimer laser applications;

FIG. 46 illustrates a method of optical imaging and inspection usinglaser dark-field at wavelengths at or below 365 nm;

FIG. 47 is an example folded 0.7 NA catadioptric objective utilizing asingle glass material;

FIG. 48 illustrates the effect of the lateral separation between raystransmitted to and received from the Mangin mirror;

FIG. 49 presents an aspect of the design that provides narrow bandaberration correction to give a 0.7 NA, long working distance,unobscured design using only fused silica;

FIG. 50 presents an aspect of the system having a folded 0.7 NAcatadioptric objective using silica and calcium fluoride to furtherincrease system bandwidth;

FIG. 51 presents an aspect of the system having an in-line or straight0.7 NA catadioptric objective employing a single glass material;

FIG. 52 presents an aspect of the system optimized for a wavelength of157 nm wherein the lenses shown are fashioned from calcium fluoride;

FIG. 53 presents an aspect of the system using two glass materials toincrease the correction bandwidth from 193 to 203 nm;

FIG. 54 presents an aspect of the image forming optics using a varifocaltwo motion zoom;

FIG. 55 presents three different magnifications possible with thevarifocal zoom;

FIG. 56 presents an aspect of the image forming optics using a singlemotion optically compensated zoom; and

FIG. 57 presents three different magnifications possible with theoptically compensated zoom.

DETAILED DESCRIPTION OF THE INVENTION

The inspection system disclosed herein employs an excimer illuminationsubsystem having advantages over non-pulsed designs. FIG. 1 illustratesa typical inspection subsystem having an illumination subsystem 101,positioning stage 102, autofocus subsystem 103, imaging subsystem 104,sensor subsystem 105, data acquisition subsystem 106, and data analysissubsystem 107. The light path travels from the illumination subsystem101 to the positioning stage 102, the imaging subsystem 104, and thesensor subsystem 105. Data passes between the autofocus subsystem 103and the positioning stage 102, between the sensor and the dataacquisition subsystem, and between the data acquisition subsystem andthe illumination subsystem and the data analysis subsystem, and betweenthe data analysis subsystem and the positioning stage.

Inspection Modes

Many different modes exist for inspecting partially fabricatedintegrated circuits and photomasks. Potential inspection modes includebright field, ring dark field, full sky, directional dark field,differential interference contrast, and confocal. These modes can beimplemented in reflection for inspecting wafers and photomasks or intransmission for inspecting photomasks. An inspection system can supportone or more of these inspection modes. In addition, an inspection modereferred to as aerial imaging can be used with photomasks to simulatethe image profile at the wafer plane.

The bright field inspection mode is similar to common microscope systemswhere a magnified view of the object is projected onto a sensor. Theadvantage of bright field imaging is that the image produced is readilydistinguishable. The size of image features accurately represents thesize of object features multiplied by the magnification of the opticalsystem. This technique can be more easily used with image comparison andprocessing algorithms for computerized object detection andclassification on patterned objects. This inspection mode is commonlyused for both wafer and photomask inspection.

The dark field inspection mode is primarily used to detect scatteringfrom edges, small particles, and irregular surfaces. For example, smoothflat areas scatter very little light resulting in a dark image. Anysurface features, particles, or objects protruding above the flat areascatter light and produce a bright. Dark field inspection modes providea large signal for small features that scatter light. This large signalallows larger sensor pixels to be used for a given feature size,permitting faster wafer or photomask inspections. Fourier filtering canalso be used to minimize the repeating pattern signal and enhance thedefect signal to noise ratio.

Many different dark field inspection modes exist including ring darkfield and directional dark field. Each uses a specific illumination andcollection scheme such that the scattered and diffracted light collectedfrom the object provides the best signal. The ring dark field inspectionmode consists of illumination and imaging pupils that do not overlap. Atypical example of this is an illumination NA that delivers light to thewafer or photomask through the high NA portion of the optical pupil. Anaperture in the imaging pupil is used to block the central portion ofthe NA used for illumination and allow scattered light collected in theouter portion of the imaging pupil to pass and form an image. Thesesystems have the advantage that features on the wafer or photomask areilluminated uniformly form all directions so features with differentorientations are equally well imaged. The NAs can also be reversed withthe illumination through the central portion of the NA and the imagingin the outer portion of the NA.

The directional dark field inspection mode can have a wide variety ofconfigurations. Each configuration is optimized for particular defecttypes. One configuration, sometimes referred to as aperture shaping,uses apertures placed at the illumination and imaging pupils. Theapertures are used to select different portions of the illumination andimaging pupils. For example, an aperture can be placed near the edge ofthe illumination pupil. This effectively delivers a small cone of lightat a high incident angle to the wafer or photomask. Another aperture orapertures can then be placed in the imaging pupil to select a desiredportion of the scattered light. For example, two apertures can be placed90 degrees to the illumination pupil aperture selecting the lightscattered sideways from features on the wafer or photomask. Many otherexamples of illumination and imaging pupil apertures can be used tooptimize for specific defect types. Another configuration, sometimesreferred to as laser directional dark field, uses one or more lasersthat illuminate the sample at high angles of incidence from outside theobjective. Often four illumination beams are chosen at 90 degree anglesfrom one another. This helps eliminate any directional dependence offeatures on the sample. A further configuration, sometimes calledinternal laser dark field, is a hybrid of the aperture shaping and thelaser directional dark field modes. In this mode a laser is injectedinto a particular location in the illumination pupil of an opticalsystem.

The full sky optical configuration is a combination of bright field andring dark field configurations. Full sky consists of using differentamounts of attenuation. The relative bright field signal and dark fieldsignal can be adjusted. This allows the detection of both bright fieldand dark field defects simultaneously using the same sensor.

The Differential Interference Contrast (DIC) inspection mode isprimarily used for its ability to resolve gradients in the topology ofobject features. The image contrast increases for increasing gradientsin the optical path. DIC mode uses a spatial shearing system with theshear distance on the order of the optical system resolution, and istypically implemented by separating the illumination into two orthogonalpolarized beams. These beams interact with the features on the objectand are the recombined before the image is formed.

The Confocal inspection mode is primarily used for its ability toresolve the topology differences of object features. Most opticalconfigurations have difficulty detecting changes in the topology offeatures. The confocal configuration discriminates between differentheights by using apertures near the illumination and imaging focus.Laser illumination can also be used to eliminate the need for theillumination aperture.

Each of the inspection modes can be implemented in reflection forinspecting wafers or photomasks, or in transmission for inspectingphotomasks. Photomask inspection has the added advantage of being ableto perform transmitted and reflected inspection simultaneously.

The Aerial Imaging configuration is typically used with transmittedlight for photomask inspection. The goal of Aerial Imaging is tosimulate the conditions present in a lithographic exposure tool. Themain advantage of this inspection mode is that only photomask defectsthat will print on the wafer will be imaged. This mode tends to have anindirect measurement of defects on the photomask. If an error isdetected in the image, the defect type causing the error can generallyonly be inferred.

Illumination

The present system employs an excimer laser illumination subsystem. Thissystem is composed of an excimer laser that is optimized for inspectionapplications and optics to relay the excimer illumination onto thesample. In addition, optical assemblies for reducing the peak power ofeach pulse, improving the spatial uniformity, and reducing specklecontrast of each pulse may be employed.

An excimer laser used in a high speed inspection system has specializedrequirements not available on commercial excimer lasers. Theserequirements affect the design of the laser. The requirements are arepetition rate high enough to support the desired data rate, longlifetime, low cost of ownership, low coherence, and stable output. Inaddition, the laser can be operated with its natural linewidth forimproved speckle smoothing or line narrowed to simplify the opticaldesign. The excimer laser distributes pulsed light energy at relativelyhigh powers, with wavelengths including approximately 308 (XeCl), 248(KrF), 222 (KrCl), 193 (ArF), 157 (F₂), and 126 nm, where such excimerlasers may include a discharge chamber containing two or more gases suchas a halogen and one or two rare gases. Other gases, including but notlimited to XeF (350 nm) may be employed with the excimer laser. Naturalbandwidths range from several nm for a 248 nm excimer to 1 pm for a 157nm laser, and bandwidths may be narrowed using dispersive componentswithin the laser cavity. Excimer lasers employed may have high dutycycles to allow for continuous operation without the need for frequentmaintenance. The excimer laser light source may include low jittercharacteristics and pulse to pulse feedback may be employed using aspecialized sensor or the image sensor itself.

It is also possible to reduce the contrast of interference and speckleby reducing the spatial coherence of the excimer laser. Traditionally,excimer lasers use plane mirror resonators. An excimer laser in thisconfiguration typically has low spatial coherence. However, it ispossible to use a cavity with curved reflective surfaces, or mirrors, tofurther reduce spatial coherence.

A further aspect of the present illumination subsystem is the ability toaddress speckle concerns and provide a system addressing peak powerassociated with energy transmission. The subsystem uses multiple beamsplitters in an arrangement that has the ability in many environments tominimize the energy variation between pulses. This system allows for aflexible setup where various combinations of plate beamsplitters andcube beamsplitters in different arrangements and geometries may be usedwhile still within the scope of the teachings of the current invention.

Typical optical delay lines can be a major source of losses. The lossesin the delay arms result from imperfect optics such as mirrors havingless than 100% reflectivity, beamsplitters with loss and unequalbeamsplitting ratios, absorption of light energy in glass materials andcoatings, and light energy scattering effects. These optical delay linelosses adversely contribute to variations in the pulse-to-pulse energyunless a method of compensation is used. In the present pulse stretchingscheme, components may be introduced between the beamsplitters tocompensate for losses in the beamsplitters, mirrors, and optical delaylines. The net result is that the pulse energies tend to be moreuniform. High efficiency within the system minimizes the requiredintroduction of compensating losses.

A schematic of an aspect of a scheme to generate four pulses is shown inFIG. 2. From FIG. 2, light energy is initially generated by an excimerlaser 201. The light energy is shown as four separate beams to moreclearly illustrate the formation of four separate pulses. In most realsituations only a single light beam would originate from the excimerlaser. The light energy from excimer laser source 201 is a pulsed lightsource. Light is transmitted toward beamsplitter 202, which splits thelight energy. The pulse reflected by beamsplitter 201 is directed to the10 ns optical delay 203, and beamsplitter 204. Beamsplitter 204 mayagain either split the beam or permit the beam to pass through. If itpasses through, it is directed to the 20 ns optical delay 205, mirror206, and to the specimen. In the case of the pulsed light energy passingthrough beamsplitter 202, said light energy contacts loss compensator207 and subsequently passes to beamsplitter 204. Loss compensator 207compensates for imperfect optical components such as the beamsplitter202 or loss in optical delay 203. In this manner, light energy reflectedby beamsplitter 202 contacts beamsplitter 204 at the same or nearly thesame energy as light energy passing through beamsplitter 202 and losscompensator 207. Similarly, light energy from beamsplitter 204 thatpasses through loss compensator 208 strikes the sample surface atapproximately the same energy as light passing the 20 ns optical delay205 and mirror 208. If the light from source 101 is polarized, mirror208 could be replaced by a waveplate and polarizing beamsplitter. Inthis manner the beams can be easily co-aligned. This mechanizationprovides for varying delays of the pulsed light energy such that lightenergy strikes the specimen surface at a desired time with relativelyuniform energies.

The design presented in FIG. 2 generates four pulses each delayed by adifferent amount of time.

The pulse passing directly through both beamsplitters has no delayintroduced, while deflecting off both beamsplitters introduces a 10nanosecond delay. 20 and 30 nanosecond delays can also be introduced inthis arrangement as shown. This introduction of delay reduces the peakpower of the pulses contacting the specimen surface.

The effects of using a design similar to the one illustrated in FIG. 2are illustrated in FIGS. 3 and 4. The system used to generate the pulsesin FIG. 4 is capable of producing eight pulses delayed by varyingamounts of time. In FIG. 4, a 532 nm laser pulse is delivered to thespecimen surface. The magnitude of the energy striking the surface is100 percent. FIG. 4 shows the multiple pulses delivered to the surface,wherein the spacing between pulses is 14.2 nanoseconds, and eight pulsesare delivered in 100 nanoseconds. The magnitude of the pulses deliveredis on the order of 12.5 percent. Thus rather than exposing the surfacewith a single large energy pulse, the surface is contacted by multiplesmaller pulses.

A scheme to create multiple pulses from a single pulse poses problemswith producing a uniform energy for the multiple pulses. This isespecially true when a large number of pulses or long delays arerequired. In addition, maintaining uniform pulse amplitudes is furthercomplicated in the UV-DUV portion of the spectrum. Optical losses tendto be very high because of increased absorption, less efficient AR andHR coatings, and increased scattering. However, even efficient opticalsystems can still suffer significant differences in pulse energies. Inthis scheme, compensators are used to add additional losses, similar tothose produced by the beamsplitters, mirrors, optical delay lines, andso forth, in order to make the pulse energy uniform.

Many different schemes can be used for compensation. A common techniqueis to use attenuation in the form of reflective or absorbing filters.The appropriate filters can be used to compensate for the losses andmake the pulse energies uniform. Continuously variable filters areavailable that allow exact matching. In addition, other techniques canbe used, such as employing a polarization based attenuator when usingpolarized light.

The optical delay line is an important component of the present system.Imaging relays or stable optical cavities are preferred because theymaintain the beam profile and stability over long optical delay paths.Many of these schemes are commonly known in the industry. Reflectivecavities such as White cells, Herriott cells, or other reflectivemultipass cells are typical examples. One major problem with these typesof multipass cells is they can be very inefficient. If long opticaldelays are necessary, many cavity round trips will be required with manymirror reflections. In the DUV-VUV spectral range, where mirror coatingsmay not be highly reflective, the efficiency of an all reflectiveoptical delay line may be unacceptable. For this reason it is desirableto employ optical delay schemes that minimize losses.

In the DUV-VUV spectral region, antireflection coatings are typicallymore efficient than HR coatings. In addition, interfaces at Brewster'sangle and TIR surfaces can have extremely low loss. The present designallows the use of novel optical delay schemes that can utilize Brewstersangle surfaces, TIR surfaces and transmissive surfaces that can be ARcoated to greatly enhance the efficiency of the optical delay scheme.One such novel optical delay scheme utilizing these types of surfaces isillustrated in FIG. 5. The system of FIG. 5 utilizes two prisms, leftprism 501 right prism 502, having total internal reflections and an ARcoated surface as an optical delay mechanism. This arrangement has theadditional advantage that the optical delay can be tuned simply byrotating the prisms about their common axis. From FIG. 5, the light beamis introduced into the arrangement and is deflected by a mirror 506 toleft prism 501, which directs light outward toward right prism 502.Right prism 502 has two TIR (total internal reflection) surfaces 503 andan AR (anti-reflective) surface 504 for directing the beam back towardleft prism 501. After a single pass through the arrangement, lightenergy exits the arrangement, shown as the output beam in FIG. 5 usingmirror 505 to direct the light energy outward. Additional methods can beused to direct the input and output beams. Examples of these methodsinclude a single mirror using the front surface for the input and therear surface for the output, or a prism using TIR and AR surfaces inmuch the same manner as prism 501 and 502. In addition, the input andoutput beams can be located in a variety of positions within the cavityto suit the particular application. This produces the necessary delayfor the system in an efficient manner. As may be appreciated, thedesired time of the delay directly affects the spacing between thevarious components.

Further delays may be obtained by creating multiple trips between thereflecting surfaces prior to passing the light energy out of thearrangement. The increase in delay by rotation of the left prism 501 andright prism 502 are shown in FIG. 6. The arrangement shown in FIG. 6 hasthe limitation that the beam is not re-imaged as it passes back andforth between the prisms. An image relay can be added to the arrangementof FIG. 6 by placing a lens or lenses between the prisms. Addition of alens or lenses provides for re-imaging such that an image may beretrieved and processed at varying points in the design, thus providingincreased control over the quality of the image received. An imagingrelay can be inserted in the optical delay arrangement as shown in FIG.8. This optical delay improves the stability and maintains the beam sizefor long optical delays. An image relay example using two lenses in anafocal telescope arrangement is shown in FIG. 7. Alternately, one ormore prism surface can be curved to act as a lens, in the case of an ARsurface, or a curved mirror, in the case of a TIR surface, for purposesof re-imaging the light.

Novel optical delay schemes utilizing TIR and Brewster's angle surfacesare also possible. One such optical delay geometry is shown in FIG. 9.From FIG. 9, input beam 901 is directed into the arrangement andredirected using a mirror 902 toward first prism 903. First prism 903directs the received beam toward second prism 904, which directs thebeam toward third prism 905. Each prism has a TIR surface and twoBrewster angle surfaces to efficiently deflect and transmit the lightenergy. Once light energy is reflected by third prism 905, it is outputas output beam 907 from the arrangement using a mirror 906. A lens orlenses can also be added to this geometry to re-image the light, eitherin the path of the light or at the entrance or exit of one of theprisms. Multiple round trips can be achieved by providing a small angleof the beam out of the plane of the drawing in FIG. 9. This will causethe beam to walk down the surfaces of the prisms with each round trip.

The system further includes the ability to reduce speckle effects intransmitted and received light. It can be shown that when a laser beamenters a diffuser at a different angle, the speckle pattern of the lightenergy leaving the diffuser also changes. This change in speckle patternfor different angles enables generation of multiple speckle patterns bymultiple beams at multiple angles when light energy passes through adiffuser. These speckle patterns can be integrated together to reducethe speckle contrast. However, in order for integration to functionproperly, each speckle pattern must arrive at the detector at slightlydifferent times. Varying arrival times of speckle patterns can beachieved by using the same optical apparatus previously described toreduce the peak power of a laser pulse. The optical apparatus, such asthat illustrated in FIG. 2, generates multiple pulses separated in timefrom a single input pulse.

The difference between using the system illustrated in FIG. 2 forreducing peak power and using the system to reduce speckle contrast isthe alignment of the optical apparatus. Typically, when multiple pulsesare generated to reduce the peak power of a single pulse, all theoptical paths are co-aligned to have the same optical axis and the samebeam position at the exit of the optical apparatus. However, forreducing the speckle contrast, it is desirable to have different anglesbetween the different optical paths. Different angles are achieved byslightly changing the angles of the mirrors and beamsplitters in theoptical apparatus. This angular change produces different angles betweeneach output pulse as the pulse exits the optical apparatus and entersthe diffuser as shown in FIG. 10. The result of using two and fourpulses to reduce the contrast of a speckle pattern is shown in FIG. 11.From FIG. 11, a typical DUV laser arrangement without the implementationof FIG. 2 having varying angles between optical paths produces a specklecontrast of 80 percent. Use of the implementation of FIG. 2 may entail,for example in a two DUV beam arrangement, light energy being directedthrough the beamsplitters and loss compensators for one channel, i.e.the 0 ns loss leg of FIG. 2, as well as the 10 ns path. Such animplementation requires redirecting at least one path of light energy,such as the energy emitting from the 10 ns delay path, so as to contactthe surface at an angle different from the 0 ns energy path in a manneras demonstrated in FIG. 10, i.e. at an offset angle from the 0 ns path.Using this type of implementation, speckle contrast may be reduced to onthe order of 56 percent. Use of four separate and summed DUV beams, suchas all four paths illustrated in FIG. 2, reduces the speckle contrast toon the order of 40 percent.

One problem with this scheme is that diffusers may not be efficient. Inthe arrangement illustrated in FIG. 2, a phase plate may be inserted inthe system instead of a diffuser to increase efficiencies. Phase plateswith multiple levels or continuous profiles can provide efficienciesapproaching 100%.

The second method for reducing speckle contrast using a single pulseemploys a grating to produce an optical delay from one side of the pulseto the other. The use of a grating to delay a portion of the pulse isillustrated in FIG. 12. Grating 1201 causes one side of the laserwavefront to be delayed in time. This delay caused by grating 1201changes across the beam making the wavefront tilt in time. In FIG. 12,the wave emanates from the light generating device (not shown) at thebottom of the illustration. The pulse has a diameter D and in thearrangement shown the left portion of the beam strikes the grating 1201and is redirected by the grating 1201 before the right half of the pulsestrikes the grating. The distance covered in a fixed period of time isthe same for the right and left side of the pulse, and thus by the timethe right side of the pulse reaches location 1202, the left side of thepulse has reached location 1203. From the illustration, the right sideof the pulse covers an additional distance L before striking grating1201. The illustration shows an approximate 45 degree angle between thepulse and grating 1201, but in practice other angles could be employedwhile still within the scope of the invention. In the illustrated 45degree angle case, the right side of the pulse covers a distance that isultimately 2 L shorter than the distance covered by the left side of thepulse. This differential in time or in distance covered produces adifferential akin to the delay produced by the implementation of FIG. 2.The resultant tilted wavefront can be used in combination with adiffuser or phase plate to reduce the speckle contrast.

From FIG. 12, the initial laser pulse will have a well defined coherencelength. After the pulse passes through grating 1201 one side of thepulse is delayed and the coherence length remains the same. The rightside of the pulse is delayed with respect to the left side by:Delay=2L=2D tan θ_(i)

where D is the diameter of the input beam and θ_(i) is the diffractionangle. This mechanization effectively breaks up the pulse into manyindependent sections that do not interfere with each other. Theseindependent sections combine in intensity to reduce the specklecontrast. The number of independent sections is equal to:

${Sections} = \frac{2\; L}{l_{c}}$

where 2 L is the maximum delay and 1_(c) is the coherence length. Theresult of the use of a grating such as that presented in FIG. 12 toreduce the contrast of a speckle pattern in shown in FIG. 13. From FIG.13, speckle contrast may be reduced from 80 percent for a single pulseto 29 percent using a grating as shown in FIG. 13.

Speckle reduction techniques using the implementation of FIG. 2 and thatof FIG. 11 may be used in combination to further reduce specklecontrast. In addition, the use of optical delays and gratings or otherredirectional or delaying elements can be used in combination with alight pipe or lens array to produce an ideal uniform illumination sourcewith low peak power and low speckle contrast. FIG. 13 illustrates theoperation and elements in a system for reducing speckle contrast. Step1301 involves generating the initial laser pulse. Step 1302 provides fortilting the pulse using a grating such as the grating 1201 presented inFIG. 12. Step 1403 comprises splitting the pulse received from thegrating and delaying the pulse using multiple exit angles. Step 1404indicates passage of the varying angle and delayed pulses through groundglass or phase plates and subsequently passing the received light energyto a light pipe or lens array in step 1405. Other combinations of thepulse delay or dividing and combining techniques disclosed herein arepossible while still within the course and scope of the invention.

The system and method described for creating multiple pulses from asingle pulse effectively increases the repetition rate of a repetitivelypulsed source. For example, if a 2 kHz excimer laser is used incombination with the system designed to create four pulses as describedin FIG. 2, the repetition rate is increased to 8 kHz. In addition, thesystem and method described for reducing the speckle contrast from asingle pulse using a grating to delay one side of a pulse with respectto the other side effectively increases the pulse length in time. It istherefore conceivable that by using both of these techniques incombination, a continuous or nearly continuous source can be producedfrom a high repetition rate source. To illustrate this, assume a laseroperating at 80 MHz with a 100 ps pulse width is used in combinationwith a system, similar to that described in FIG. 2, designed to create32 pulses with the appropriate delays, the repetition rate iseffectively increased to 2.6 GHz. The pulse separation of the 2.6 GHzsource is around 400 ps. Now if the 100 ps pulse can be stretched to 400ps, the source can be considered continuous. Using a grating at asymmetric 45 degree angle, the 100 ps pulse can be stretched to 400 psusing a beam 2.4 inches in diameter. One potential problem with thisapproach is the spectral dispersion created by the grating. This can beeliminated by adding a second grating. This eliminates the spectraldispersion while maintaining the optical delay from one side of thepulse to the other.

Positioning

The positioning subsystem for an excimer laser based inspection systemhas several desirable aspects. Some of the desirable aspects are highspeed positioning of the specimen, rotation capability for alignment ofthe specimen, translation along the optical axis for focusing of thespecimen, and position output for are synchronizing with the excimerlaser.

High speed positioning of the specimen can be achieved using a precisionstage. Stages of this type typically use air bearings on a precisionsurface, including but not limited to granite, to define the motion.High speed motion is most often achieved using one or more linearmotors. It is also possible to produce high speed motion using a leadscrew with servo motors. The excimer laser based inspection system mayhave loose requirements for vibration and speed variation in the stageif only a single pulse is used for illuminating the sample. This isbecause the illumination pulse typically lasts only 10 nanoseconds to 1microsecond or so with pulse stretching. A short exposure time mayeffectively make stage may appear to be stationary. Small variations inthe stage position may be within the overlap area of the individualexposure frames.

There are several desirable scanning options for a stage used for highspeed inspection. The primary method commonly used to inspect patternedsamples like wafers and photomasks is to use a raster scan. In thistechnique the stage moves the sample across the imaging subsystem fieldof view in one direction. The stage is then incremented in theorthogonal direction and the stage moves the sample across the imagingsubsystem field in the opposite direction. This is repeated until thedesired area of the sample is inspected. It is also possible to move thesample in an R-theta scan. In this technique the sample is rotatedacross the imaging subsystem field of view. As one rotation is completethe radius is increased until the desired area of the sample isinspected. The sample can be stepped I the radial direction orcontinuously moved to create a spiral inspection path.

It may also be desirable to have rotation capability on a rasterscanning positioning subsystem. This allows features on the sample suchas straight lines or objects oriented in rows or linear patterns to bealigned with the scanning direction. As the sample is scanned the lineor pattern will maintain the same position on the image sensor. This cansimplify and speed up the high speed data analysis.

There are two approaches to synchronizing the stage and excimer laser.In one approach, the excimer laser is synchronized with the stageposition. An excimer laser can be triggered with high accuracy tocoincide with the desired illumination area on the sample. Triggeringrequires the stage have the ability to provide accurate position outputusing encoders, distance measuring interferometers, or other positionsensing devices. In another approach the stage is synchronized to theexcimer laser firing. Synchronization according to this approachrequires the stage speed to be varied so the desired stage positioncoincides with the arrival of the laser pulse.

Focusing the sample can be performed using a stage having the ability tomove along an axis parallel to the optical axis and orthogonal to thescanning plane. Focusing often has several desirable aspects. Focusingmay be fast enough to maintain focus during a high speed scan. This mayrequire operation at 1000 Hz or higher. The resolution may be highenough to stay substantially within the depth of focus of the opticalsystem. For high NA, short wavelength systems, this is often less than50 nm. This requires a high resolution motion system such as a PZTsystem.

Imaging

The design of the imaging element of the system may be a high numericalaperture (NA) system having a large field and accommodating the narrowband excimer laser light source to support a variety of imaging modes.Single shot imaging may not require use of a TDI sensor, while multipleshot imaging addresses issues of blurring, is synchronized betweenshots, and is smoothed via optics, laser, sensor variation, and peakpower techniques. Broadband and narrowband imaging is supported, wherebroadband may include diffractive optics and the use of two materialsfor all refractive imaging. The optical design may utilize more than onewavelength for autofocus, optics with an external pupil or Fourierplane, and zoom capability. Purging and contamination control of theoptics may be provided, such as being oxygen free for 157 nm lightenergy sources. Referring now to FIG. 17, shown is a figure illustratingan apparatus that combines the functions of several imaging systems.This apparatus, based on a high NA catadioptric optical design, is anarrow band optical system having an NA greater than 0.90 and is highlycorrected for low and high order monochromatic aberrations. The systemmay have a numerical aperture of greater than 0.65. Preferably thenumerical aperture is greater than 0.90. The field size preferablyranges from 0.5 to 2.0 mm.

The high NA catadioptric optical system disclosed herein may have an NAin air up to 0.99 and a field size of greater than 1 mm. Such a systemhas relaxed manufacturing tolerances and only requires a single glassmaterial. Use of a single glass material in the catadioptric system isvery advantageous when the system is optimized for the spectrum below300 nm because only a few glasses with high transmission are available.

The high NA catadioptric objective illustrated may be used and optimizedfor light beams having different wavelengths, from the infrared to thedeep ultraviolet. For example, in the ultraviolet spectrum, light beamshaving wavelengths of 193 nm, 213 nm, 244 nm, 248 nm, 257 nm, 266 nm,and so forth are possible using the concepts disclosed herein, withadjustments that would be apparent to those of ordinary skill in theart. For wavelengths from 110–200 nm, fluoride glasses may be usedbecause of their advantageous transmission properties.

The illustrated catadioptric optical system provides high qualityimaging performance at numerical apertures (NAs) up to 0.99. This NArange represents the capability to illuminate and image at very highangles of incidence. The relationships between the numerical aperture inair and the angle of incidence to the sample are that:NA=n*sin(angle of incidence)

where the index n has a value of 1.000 for air.

The following table summarizes the relation between NA and the angle ofincidence in air:

NA Angle of incidence (in air) (degrees) 0.90 64 0.91 66 0.92 67 0.93 680.94 70 0.95 72 0.96 74 0.97 76 0.98 79 0.99 82

FIG. 17 is a 0.97 NA design having a 1 millimeter field size. The designis optimized for a 266 nm wavelength and uses only fused silica. Thesystem has a Strehl ratio of 0.98 and can resolve on the order of 6,000spots along one dimension of the 1 mm field. As will be discussed below,increased field sizes are achievable using this system. For a 1.5 mmfield size, the Strehl ratio decreases to 0.95 and the number ofresolvable spots along one dimension of the field increases toapproximately 9,000. For a 2.0 mm field size, the Strehl ratio decreasesto 0.85 and the number of resolvable spots along one dimension of thefield increases further to approximately 12,000. An increase in thenumber of resolvable spots increases the efficiency for a givenconfiguration and faster inspection of an object is possible. Possibleapplications of this optical system include wafer and photomaskinspection, material masking and cutting operations, UV lithography,biological microscopy, metallurgical microscopy and others.

Note that the elements of FIG. 17 are drawn to scale, with the numberand line in the upper left corner indicating a distance in millimeters,here 25 millimeters. This notation is used throughout several figuresincluded herein.

The design in FIG. 17 includes a catadioptric group 1701 proximate to anintermediate image, and a focusing optics group 1702. Light scattered,diffracted, and reflected by the object 1704 is collected by thecatadioptric group 1701 which forms an intermediate image 1707. Thefocusing optics group 1702 corrects for the aberrations present in theintermediate image. The working distance of the design presented in FIG.17 is approximately 0.5 millimeters, or a distance of approximately 0.5millimeters exists between the object 1704 and the single refractiveelement 1705. The central obscuration is limited to 10 percent of thebeam diameter.

From FIG. 17, catadioptric group 1701 includes near flat reflector witha reflective surface coating 1705 and a dome-shaped reflector 1706. Thenear flat reflector with a reflective surface coating 1705 can be aparallel fused silica plate having zero power. Focusing optics group 202includes first focusing lens element 1709, second focusing lens element1710, third focusing lens element 1711, fourth focusing lens element1712, fifth focusing lens element 1713, sixth focusing lens element1714, seventh focusing lens element 1715, and eighth focusing lenselement 1716. The focusing optics group 1702 corrects for high orderspherical aberration and coma. The focusing optics group design uses afield lens concept originally developed by Offner, but the Offner designonly works for systems having near zero field size. The large fields inthese objective designs require unique optimization techniques. Thecomplexity and shapes of the lenses in the focusing group 1702 becomeextremely critical for high NA values, such as those exceeding 0.90, andfor large field sizes.

The ultra high NA disclosed allows for a variety of flexibleillumination schemes. Illumination angles from 0 to 85 degrees can beimplemented, thereby allowing maximum flexibility when choosing anillumination angle.

The catadioptric optical system has two primary methods forillumination. First, light energy can enter through the lenses of theoptical system at angles from 0 to the angle defined by the objectiveNA. Second, for oblique illumination at angles from about 12 to 85degrees, the preferred method is by introducing the illumination throughan aperture in the coating of the dome-shaped reflector as shown in FIG.18. As a result, illuminating light only passes through a few surfaces,thereby reducing the potential for multiple surface reflection andscattering to reach the imaging detector. As shown therein, a beamintroduced through an aperture in the mirror coating 1706 maintains thesame angle with respect to the wafer 1704 when it exits the objective.In other words, illumination introduced at an angle of 85 degrees to thenormal 1801 exits the fused silica single refractive element 1705 at thesame 85 degree angle. For the lens design shown in FIG. 17, thedimensions for the catadioptric group to provide uniform angles ofincidence are as shown in FIG. 19. The thickness of fused silica nearflat reflector 1705 is 14.51 mm with a diameter of 170 mm. Thedome-shaped reflector 1706 has an inner radius of curvature of 75.8 mm,and an outer radius of curvature of 91.5 mm. The distance on thecenterline between the inner edge and the outer edge of the dome-shapedreflector 1706 is 12 mm, and the distance from the inner edge of thedome-shaped reflector to the fused silica near flat reflector 205 is47.46 mm.

If the design is reoptimized by changing the thickness or radius ofcurvature of the near flat reflector 1705 or the inner radius of thedome-shaped reflector 1706, the thickness and/or outer radius ofcurvature 1802 of dome-shaped reflector 1706 may be modified to preservethe angle of illumination. Under oblique illumination, in addition topreserving the angle when a beam enters and exits the objective, thecatadioptric elements do not introduce any power to the illuminationbeam. A collimated beam of light energy entering through an aperture ina mirror coating will be collimated when it exits the objective. Theaperture in coating of the dome-shaped reflector 1706 may include a slitof non-mirrored surface, single holes of non-mirrored surface, aphysical hole in the mirror, a partial mirror coating, or a coating thatselectively transmits the wavelength of interest. In addition, multiplebeams at multiple angles may also be used for illumination. For example,using oblique illumination, two beams separated azimuthally byapproximately 90 degrees may be used for illumination to minimizeshadowing effects.

The catadioptric imaging system effectively collects light scattered,diffracted, and reflected at different angles by the object and mapsthese scattering angles to a plane. This plane is located at the pupilof the system and each position on this pupil plane corresponds to aposition on the dome-shaped reflector 1706. Each location in the pupilplane corresponds to different scattering angles, and apertures placedat this pupil plane can be used to limit the range of scattering anglesreaching the image detector. This pupil plane roughly corresponds to theFourier plane of the object. Such a system supports collection NAs up to0.99 for illumination angles between 0 and 85 degrees.

The wide range of illumination and collection angles possible with thiscatadioptric optical system allows it to support multiple imaging modes.These modes include, but are not limited to, variable NA bright field,full sky, ring dark field, inverted ring dark field, directional darkfield, double dark field, central dark ground, Manhattan geometry,confocal bright field, confocal dark field, as well as conoscopicimaging. Many other schemes are also possible in which the illuminationangle is between 0 and approximately 85 degrees and the collection anglebetween 0 and approximately 82 degrees.

FIGS. 25 through 28 illustrate illumination of the specimen, such as awafer surface, using dark field illumination. FIG. 25 is side view of adouble dark field design system 200 that illuminates the object 201using laser beam 202 directed at a relatively low angle of incidence.Collectors 203 and 204 are mounted at different angles from the laserbeam 202, typically 90 degrees. FIG. 26 illustrates a top view of thesystem with the collectors 203 and 204 mounted 180 degrees from oneanother and 90 degrees from the laser 202. Variations of these anglesare possible. This provides enhanced collection capability and allowsdetection of particular object faults. FIG. 27 illustrates a variationof a central dark ground imaging system 300, wherein the laser beam 301passes through the collector 302 at an approximately perpendicular angleto the object 303. The light beam strikes the object and is diverted,depending upon the features encountered, toward various collectorsmounted about the object. Four collectors 305–308 in FIG. 28 have beenemployed in the past, each at an angle 90 degrees from the nearestcollectors, as shown in FIG. 3 b. Different numbers of collectors may beused at various angles depending upon the type of object scanned and thedefects anticipated.

Variable NA bright field mode may be employed using the conceptsdisclosed herein. The illumination of the object in this system issimilar to that in a standard microscope. The light may be injected intothe optical system using a beam splitter and then projected through thefocusing group 1702. Light energy then passes through the aperture inthe dome-shaped reflector 1706 and strikes the reflective surface onnear flat reflector 1705. Light energy then passes back through the nearflat reflector 1705, striking dome-shaped reflector 1706, passing onceagain through the near flat reflector 1705 to illuminate object 1704.Note that the right surface of the near flat reflector 1705 shown inFIG. 17 is necessarily clear, or non-opaque, at the center portion suchthat light may contact object 1704 but has a reflective interior surfaceoutside the center portion to provide reflectivity. Variableillumination NA can be obtained by using an aperture at the objectivepupil plane, in the collimated range of the objective, or in separateoptics before the beam splitter. The light passes through the objectiveand is then scattered, diffracted, and reflected from the object. Thelight from the object returns through the objective and through the beamsplitter. The variable NA of the light from the object can be obtainedby using an aperture at the objective pupil plane or in the collimatedrange of the objective. An image is formed on a detector usingadditional lenses as described below. Narrow band illumination may beemployed. The bright field illumination could be a narrow band laser ora broad band source with a narrow band filter. To reduce the problem ofspeckle and interference from narrow band light, a moving ground glassor some other technique to introduce random phase may be placed into thebeam before light enters the objective. Other speckle reductiontechniques are disclosed below.

Alternately, the system can operate in full sky mode. This mode is avariation of the variable NA bright field mode described above. Full skyuses the same type of variable NA illumination, however, the NA of thelight collected from the object should be as large as possible. Imagingin this mode collects as much light as possible coming from the object,especially in the higher angles. Full sky mode can minimize contrastvariations introduced by grain and rough films.

The system can also operate in ring dark field mode. This is a standardtype of dark field imaging where the illumination angles are limited toa high NA ring and the imaging angles are limited to the NAs less thanthose used by the ring illumination. The ring illumination can beinjected into the optical system using a beam splitter. The ringillumination can be formed by a ring reflector or by separateillumination optics previous to a beam splitter. To form the high NAring in the separate illumination optics an aperture can be placed at anequivalent pupil plane. This method of forming the ring illumination canhave the aperture block a substantial portion of the light. To avoidthis, the ring can be formed by using one or more axicons, a diffractiveoptic, a holographic optic, a segmented optic, combinations of thesedevices, and so forth. The low NA imaging can be obtained by placing anaperture at the pupil plane of the objective to limit the anglesreaching the detector.

Alternately, the system can operate in inverted ring dark field mode.This mode is the inverse of ring dark field mode and uses the low NAsfor illumination and a high NA ring for imaging. Variable low NAillumination can be obtained by using a low NA spot mirror or separateillumination optics before the beam splitter as described in thevariable NA bright field section above. High NA imaging can be obtainedby placing an aperture at the pupil plane of the objective or in thecollimated range to limit the angles reaching the detector.

The system can also operate in directional dark field mode using obliqueillumination, as shown in FIG. 20. Such an implementation can collectscattered and diffracted light from the object at variable NAs from thenear normal portion of light to the full angular range of thecatadioptric objective. In the directional dark field mode, the systemilluminates the object by injecting the illuminating beam through anaperture in the mirror coating shown in FIG. 20. Illumination source2001 emits a beam of light to lens 2002, which collimates the beam andtransmits the beam to mirror 2003. Mirror 2003 directs the collimatedbeam through the dome-shaped reflector 1706, striking the near flatreflector 1705 and the object 1704. The specular reflection from object1704 is transmitted out of the system through another aperture in thecoating of the dome-shaped reflector 1706. The scattered and diffractedlight from the object 1704 is collected by the dome-shaped reflector1706. The dome dome-shaped reflector 1706 then reflects the light tolens mirror element 1705 which in turn reflects the light to the imaginglenses in the system. Limiting the collection angles of the scatteredand diffracted light is accomplished by using an aperture in the pupilplane or in the collimated range of the objective.

Apertures of any size may be used from large apertures thatsubstantially transmit the full NA of the system to small apertures thatsubstantially transmit only the near normal light. FIG. 21 is an exampleof a pupil aperture 2101 that allows imaging light to pass through thecentral region 2102 and blocks light outside this area with annularblock. Such an aperture can be used to limit collection angles to thoseless than the illumination angle.

The system can further operate in double dark field mode using theoblique illumination and collecting the near 90 degree azimuthalportions of scattered and diffracted light. In the double dark fieldmode, the system illuminates the object by injecting the illuminationthrough an aperture in the mirror coating as is done in the directionaldark field case. The system uses an aperture in the pupil plane or inthe collimated range to limit the collection to the near 90 degreeazimuthal portions of scattered and diffracted light. Such a collectionaperture may be as shown in FIG. 22. The aperture 2201 illustrated inFIG. 22 has two apertures 2202 which collect the near 90 degreeazimuthal scattering of a double dark field system.

Additionally, the system can operate in the central dark ground modeusing normal illumination illustrated in FIG. 23. In FIG. 23,illumination source 2301 emits a beam of light to lens 2302, whichcompensates for the power in the focusing optics group 202 and transmitsthe beam to mirror 2303. Mirror 2303 directs the beam toward a spotmirror 2304, which directs the beam through the focusing optics group1702 and into the catadioptric group 1701. The beam strikes the nearflat reflector 1705 at an angle from approximately 0 to 12 degrees fromthe normal of the near flat reflector 1705 then strikes the object 1704.The scattered, diffracted, and reflected light from the object 1704 iscollected by the dome-shaped reflector 1706. The dome-shaped reflector1706 then reflects the light to lens mirror element 1705 which in turnreflects the light to the imaging lenses in the system. In this mode thespecular reflection from the object is blocked and remaining portions ofthe scattered and diffracted light are transferred to the detector. Thespecular reflection can be blocked by the spot mirror 2304 or by anaperture placed in the pupil plane or the collimated range.

The system can also operate in the Manhattan geometry mode. This modecan use normal illumination as described in the central dark ground modeor oblique illumination as described in the directional dark field mode.The Manhattan geometry uses high angle light collection from fourdifferent quadrants. An aperture that provides this type of collectionmay be as shown in FIG. 24. FIG. 24 is a filter 2401 having fourapertures 2402 for capturing high angle scattering in portions of thefour separate quadrants. Such a filter may be used in connection with asystem as shown in FIGS. 27 and 28.

Additionally, the system disclosed herein can operate in the brightfield confocal imaging mode. This mode takes advantage of the shortdepth-of-focus obtainable by using a high NA objective and shortwavelength illumination. Bright field confocal mode illuminates theobject with a single point or a line focus. The illumination spot on theobject is then imaged through an aperture in front of a detector. Thisaperture and detector can be a pinhole and a single point detector, inthe case of a single point focus, or a slit and a linear detector array,in the case of a line focus. The object, illumination spot, aperture, ora combination thereof is then scanned to collect information about anarea on the object being examined.

Additionally, the system can operate in the dark field confocal imagingmode. Dark field confocal imaging mode also takes advantage of the shortdepth-of-focus obtainable by using a high NA objective and shortwavelength illumination. This is a unique imaging mode is made possibleby the high NA diffraction limited illumination. High NA ringillumination produces a diffraction limited spot or line and theremaining NA can support diffraction limited imaging. For example, theillumination can occupy a ring from 0.9 to 0.97 NA and the NA up to 0.9can be used for imaging. The illumination spot on the object is imaged,using an NA that is less than the illumination NA, through an aperturein front of a detector. This aperture and detector can be a pinhole anda single point detector, in the case of a single point focus, or a slitand a linear detector array, in the case of a line focus. The systemscans the object, illumination spot, aperture, or a combination thereofto collect information about an area on the object being examined.

The system can alternately operate in the conoscopic mode. In this modeoblique illumination is used as described above in the section ondirectional dark field or normal illumination can be used as describedin relation to central dark ground mode. In this mode lenses are notrequired to form an image on a detector. The light at the pupil plane orin the collimated range of the objective can be placed directly on asingle detector, multiple detectors, or a detector array. An aperturelimiting the range of angles reaching the detector can also be used atthe pupil plane or in the collimated range prior to the detector. Theportions of the pupil plane that are most sensitive to the features ofinterest can be selected for detection. This signal may then be comparedto other similar signals form similar objects to detect changes in thefeatures on the object.

As may be appreciated from the previous paragraphs, the conceptdisclosed herein is that multiple imaging modes can be implemented usinga single optical system or machine in connection with the excimer laserillumination device. The ultra high NA disclosed for illumination andcollection permits the implementation of imaging modes using the sameoptical system, thereby allowing optimization of imaging for differenttypes of defects or samples. Illumination angles from 0 to 85 degreescan be easily implemented, thereby allowing maximum flexibility whenchoosing an illumination angle. Collection angles from 0 to 82 degreesare possible.

Further, it should be noted that oblique dark field illumination throughthe dome-shaped reflector in the modes disclosed herein does notinterfere with image pupil filtering. This dark field illumination canbe achieved through an aperture in the mirror coating with the entranceangle being the same angle striking the object. This feature permits theillumination and imaging pupils to be separate from one another, andthus the illumination and imaging pupil do not interfere with oneanother. The unique oblique illumination scheme used in the differentdark field modes renders the catadioptric system disclosed herein muchmore flexible with respect to desired illumination schemes andaperturing and filtering techniques.

FIGS. 21, 22, and 24 are some examples of different pupil apertures thatmay be used to select different imaging modes. As with all modeselection apertures illustrated, these may be fabricated from a sectionof clear glass with appropriate portions, such as annular block 2103,screened out using an opaque material, such as paint or othernon-transparent material. Other means of forming such filters, such asusing metal or composite material, may be used while still within thescope of the current system.

A liquid crystal device, micro-mirror array, or some other addressablearray device can be used to segment the pupil. For example, a liquidcrystal array can be placed at the pupil plane. Portions of the arraycan be made opaque and other portions transparent to correspond to thedesired pupil aperture, such as those in FIGS. 21, 22, and 23.

The dimensions for the system illustrated in FIG. 17 are as follows,where the surface numbers 0 through 26 track the surfaces and gaps thelight passes through and reflects off in performing the differentimaging modes:

This is an all fused silica design with a 0.97 NA, 1.0 mm field size,and a 15.46 mm focal length. This design is for use at a wavelength of0.266 micrometers where the index of fused silica is 1.499776.

Aperture Surface Radius Thickness [element] (mm) (mm) (mm) RadiusMaterial 0 — 1.0917e+20 3.5297e+18 Air 1 [3216] −33.497056 5.68968718.000000 Fused Silica 2 [3216] −32.189281 0.100000 19.000000 Air 3[3215] 196.880765 4.000000 19.000000 Fused Silica 4 [3215] 25.6845726.560210 17.000000 Air 5 [3214] 357.899464 5.000000 19.000000 FusedSilica 6 [3214] −90.078811 0.100000 19.000000 Air 7 [3213] 24.03781912.750000 20.500000 Fused Silica 8 [3213] 432.836029 5.920196 20.500000Air 9 [3212] −40.013173 3.250000 19.500000 Fused Silica 10 [3212] — —20.500000 Air 11 [3208] — — 35.000000 Aperture stop 12 — 10.62412035.000000 Air 13 [3211] 26.394355 20.000000 17.500000 Fused Silica 14[3211] 19.724315 4.038458 12.000000 Air 15 [3210] 128.412673 9.50854812.500000 Fused Silica 16 [3210] −24.741473 0.100000 12.500000 Air 17[3209] 21.128369 4.000000 9.5000000 Fused Silica 18 [3209] −155.17938210.50000 9.5000000 Air 19 [3206] — 47.457785 0.122526 Air 20 [3205] —14.508002 58.000000 Fused Silica 21 [3205] — −14.508002 58.000000Reflect. 22 [3205] — −47.457785 58.000000 Air 23 [3206] 75.78608647.457785 64.000000 Reflect. 24 [3205] — 14.508002 58.000000 FusedSilica 25 [3205] — 0.5000000 58.000000 Air 26 — 0.500543

Surface 26 represents the gap distance between near flat reflector 1705and object 1704. Note that surface 11 represents the aperture allowinglight to pass from surface 10 on element 1712, and surface 12 representsthe size of the air gap between within focusing group 1702.

An alternative aspect of this system may include higher NA values, anexample of which is illustrated in FIG. 29. This aspect of the system isoptimized for use at 0.98 NA. Note that the system of FIG. 29 uses thesame basic components and configuration as that shown in FIG. 17, usinga 0.98 NA catadioptric group 2901 and a 0.98 NA focusing optics group2902. As may be appreciated from the figure and the following table, thesystem components or surfaces are similar to the 0.97 NA system, withsome slightly differing surface curvatures and spatial separationsbetween the elements.

Dimensions for the system of FIG. 29 are as follows (with surfacenumbering similar to that from the table and drawing of FIG. 2):

This aspect of the system is an all fused silica design with a 0.98 NA,1.0 mm field size, and a 15.31 mm focal length. This design is for useat a wavelength of 0.266 micrometers where the index of fused silica is1.499776.

Aperture Surface Radius Thickness [element] (mm) (mm) (mm) RadiusMaterial 0 — 1.0917e+20 3.5661e+18 Air 1 [2916] −34.690724 5.00000018.000000 Fused Silica 2 [2916] −32.508483 0.100000 19.000000 Air 3[2915] 251.168193 4.000000 19.000000 Fused Silica 4 [2915] 25.2626576.670812 17.250000 Air 5 [2914] 512.120638 5.000000 19.000000 FusedSilica 6 [2914] −85.950636 0.100000 19.000000 Air 7 [2913] 24.03021013.002285 21.000000 Fused Silica 8 [2913] 386.838892 6.258149 21.000000Air 9 [2912] −38.824067 3.250000 19.750000 Fused Silica 10 [2912] — —20.500000 Air 11 [2908] — — 35.000000 Aperture stop 12 — 13.61582535.000000 Air 13 [2911] 28.596934 20.000000 18.000000 Fused Silica 14[2911] 20.398707 3.564297 12.500000 Air 15 [2910] 53.168289 8.56068513.500000 Fused Silica 16 [2910] −25.471785 0.100000 13.500000 Air 17[2909] 24.150867 3.564297 10.000000 Fused Silica 18 [2909] −265.29318210.00000 10.000000 Air 19 [2906] — 47.439408 0.422526 Air 20 [2905] —14.890298 60.000000 Fused Silica 21 [2905] — −14.890298 60.000000Reflect. 22 [2905] — −47.439408 60.000000 Air 23 [2906] 75.80846047.439408 60.000000 Reflect. 24 [2905] — 14.890298 66.000000 FusedSilica 25 [2905] — 0.5000000 60.000000 Air 26 — 0.500549

An additional aspect of the system is optimized for use at 0.99 NA. Notethat the system of FIG. 30 uses the same basic components andconfiguration as that shown in FIG. 17, using a 0.99 NA catadioptricgroup 3001, and a 0.99 NA focusing optics group 3002.

As may be appreciated from the figure and the following table, thesystem components or surfaces are similar to the 0.97 NA system, withsome slightly differing surface curvatures and spatial separationsbetween the elements.

This aspect of the system is an all fused silica design with a 0.99 NA,1.0 mm field size, and a 15.15 mm focal length. This design is for useat a wavelength of 0.266 micrometers where the index of fused silica is1.499776.

Dimensions for the system of FIG. 30 are as follows (with surfacenumbering similar to that from the table and drawing of FIG. 17):

Aperture Surface Radius Thickness [element] (mm) (mm) (mm) RadiusMaterial 0 — 1.0917e+20 3.6025e+18 Air 1 [3016] −35.203096 5.00000018.000000 Fused Silica 2 [3016] −32.502313 0.100000 19.000000 Air 3[3015] 222.523869 4.000000 19.000000 Fused Silica 4 [3015] 24.4205156.583213 17.250000 Air 5 [3014] 262.694007 5.000000 19.000000 FusedSilica 6 [3014] −125.012490 0.100000 19.000000 Air 7 [3013] 24.10596113.752435 21.000000 Fused Silica 8 [3013] 778.092175 6.309114 21.000000Air 9 [3012] −38.092856 3.250000 20.000000 Fused Silica 10 [3012] — —21.000000 Air 11 [3008] — — 35.000000 Aperture stop 12 — 15.42385635.000000 Air 13 [3011] 34.922533 20.000000 18.500000 Fused Silica 14[3011] 22.960530 3.322518 13.500000 Air 15 [3010] 50.507026 7.00000013.750000 Fused Silica 16 [3010] −24.636821 0.100000 13.750000 Air 17[3009] 24.316233 4.000000 11.000000 Fused Silica 18 [3009] 2052.110.50000 11.000000 Air 19 [3006] — 47.541690 0.122526 Air 20 [3005] —15.730439 64.000000 Fused Silica 21 [3005] — −15.730439 64.000000Reflect. 22 [3005] — −47.541690 64.000000 Air 23 [3006] 76.14372247.541690 68.000000 Reflect. 24 [3005] — 15.730439 64.000000 FusedSilica 25 [3005] — 0.5000000 64.000000 Air 26 — 0.500832

Yet another aspect of the system is optimized for use at 0.97 NA with anapproximately 4 mm field of view. Note that the system of FIG. 31 usesthe same basic configuration as that shown in FIG. 17, using acatadioptric group 3101 and a focusing group 3102. As may be appreciatedfrom the figure and the following table, the system components orsurfaces are similar to the previous 1 mm field size systems, with someslightly differing surface curvatures and spatial separations betweenthe elements.

An example of an imaging system having increased field size isillustrated in FIG. 31. As shown therein, nine lenses 3108–3117contribute with a catadioptric group to increase field size to 4.0 mmfor a 0.97 NA.

Aperture stop 3112 is located halfway through the lens arrangement. FromFIG. 31, the ray spacing is unequal in the middle of the design,although the rays become equally spaced at the left side of the lensarrangement. This illustrates the one-to-one correspondence between therays in the dome and the output plane. This ray spacing provides higherorder correction capability. As the lens arrangement causes the rayspacing to be unequal on the strongly curved ninth surface (left surfaceon lens 3113), the rays encounter more spherical aberration on the ninthsurface than normal, towards the edge of the aperture, and therebyprovide the higher order spherical aberration needed to compensate forthe catadioptric elements. The system illustrated in FIG. 31 has aStrehl ratio ranging from 0.50 to 0.73 over the 4.0 mm field size.

This aspect of the system is an all fused silica design with a 0.97 NA,4.0 mm field size, and a 17.53 mm focal length. This design is for useat a wavelength of 0.266 micrometers where the index of fused silica is1.499776.

Dimensions for the system of FIG. 31 are as follows (with surfacenumbering similar to that from the table and drawing of FIG. 17):

Aperture Surface Radius Thickness [element] (mm) (mm) (mm) RadiusMaterial 0 — 1.0000e+20 1.1429e+19 Air 1 [3116] −49.313195 7.00000026.000000 Fused Silica 2 [3116] −48.825209 0.100000 26.000000 Air 3[3115] 75.954248 5.500000 25.000000 Fused Silica 4 [3115] 34.3649085.563965 21.500000 Air 5 [3114] 80.813499 17.867130 24.000000 FusedSilica 6 [3114] 39.762098 10.928315 21.500000 Air 7 [3113] 102.0369338.0000000 26.000000 Fused Silica 8 [3113] −255.018249 0.100000 26.000000Air 9 [3112] 29.518372 21.076921 27.000000 Fused Silica 10 [3112]2.8375e+03 1.775488 27.000000 Air 11 — — 40.000000 12 [3111] — 6.14071240.000000 Air Fourier plane 13 [3110] −43.892944 39.000220 22.000000Fused Silica 14 [3110] −48.033761 8.316604 24.000000 Air 15 [3109]106.289831 6.000000 16.000000 Fused Silica 16 [3109] −68.034207 0.10000016.000000 Air 17 [3108] 34.181630 4.500000 14.000000 Fused Silica 18[3108] −1.0338e+04 12.00000 14.000000 Air 19 [3107] — 55.056163 0.138656Air 20 [3105] 5.4047e+03 19.061584 67.898763 Fused Silica 21 [3105] —−19.061584 67.898763 Reflect. 22 [3105] 5.4047e+03 −55.056163 67.898763Air 23 [3106] 89.730011 55.056163 75.820285 Reflect. 24 [3105]5.4047e+03 19.061584 67.898763 Fused Silica 25 [3105] — 0.500000067.898763 Air 26 [3104] — 2.005791

Yet a further aspect of the current design provides an external pupilplane conjugate to the internal pupil plane of the objective as shown inFIG. 32. This pupil plane will exactly correspond to the Fourier planeof the object because it is also in the collimated range of theobjective. This external pupil plane allows for improved access to theFourier plane for filtering and aperturing. The pupil plane is relayed,using a series of lenses 3209–3218 to an external plane 3208. The designhas a 0.97 NA, 1.0 millimeter field diameter, and 0.5 millimeter workingdistance. The relayed pupil arrangement shown has a Strehl ratio greaterthan 0.90 and in the configuration shown a focal length of 12.37millimeters.

The system of FIG. 32 has the following dimensions:

Aperture Surface Radius Thickness [element] (mm) (mm) (mm) RadiusMaterial 0 — 1.0000e+10 6.4667e+08 Air 1 — — 7.5000000 Fourier Plane 2[3208] — 62.515852 7.5000000 Air 3 [3218] 73.034166 5.000000 14.000000Fused Silica 4 [3218] −93.305910 91.324812 14.000000 Air 5 [3217]21.680281 4.0000000 8.5000000 Fused Silica 6 [3217] 51.554023 43.4015288.5000000 Air 7 [3216] −10.644592 20.000000 8.5000000 Fused Silica 8[3216] −19.624195 77.871853 14.000000 Air 9 [3215] 225.891254 4.000000018.000000 Fused Silica 10 [3215] 28.200709 5.753232 17.500000 Air 11[3214] 383.158596 6.000000 19.000000 Fused Silica 12 [3214] −82.1437520.100000 19.000000 Air 13 [3213] 26.7469644 14.000000 21.500000 FusedSilica 14 [3213] −185.222466 5.859221 21.500000 Air 15 [3212] −36.6649804.000000 20.500000 Fused Silica 16 [3212] −135.155561 20.36027921.000000 Air 17 [3211] 28.058433 20.00000 18.000000 Fused Silica 18[3211] 17.433830 4.434055 12.500000 Air 19 [3210] 34.161468 7.50000012.500000 Fused Silica 20 [3210] −24.210328 0.100000 12.500000 Air 21[3209] 22.396238 3.500000 10.000000 Fused Silica 22 [3209] 81.18525711.50000 10.000000 Air 23 [3207] — 48.014929 0.122526 Air 24 [3205] —15.264241 62.000000 Fused Silica 25 [3205] — −5.264241 62.000000Reflect. 26 [3205] — −48.014929 62.000000 Air 27 [3206] 77.02924848.014929 66.000000 Reflect. 28 [3205] — 15.264241 62.000000 FusedSilica 29 [3205] — 0.5000000 62.000000 Air 30 [3204] — 0.501766

Still a further aspect of the current design also provides an externalpupil plane conjugate to the internal pupil of the objective as shown inFIG. 33. This is an all fused silica design with a 0.98 NA, 2.0 mm fieldsize, a 7.653 mm focal length, and 0.75 millimeter working distance.This design is for use at a wavelength of 0.266 micrometers where theindex of fused silica is 1.499776.

The system of FIG. 33 has the following dimensions:

Aperture Surface Radius Thickness [element] (mm) (mm) (mm) RadiusMaterial 0 — 1.0000e+20 1.3065e+19 Air 1 — — 7.50000 Fused Silica 2 —56.10343 7.50000 Air 3 172.92817 6.50000 17.00000 Fused Silica 4−51.15640 1.00000 17.00000 Air 5 36.69504 7.00000 17.00000 Fused Silica6 −101.81708 9.33307 17.00000 Air 7 −40.11542 3.00000 13.00000 FusedSilica 8 −131.54232 32.30953 13.00000 Air 9 −10.54200 5.29845 8.00000Fused Silica 10 −13.00226 126.38070 10.00000 Air 11 −1615.27308 4.5000029.00000 Fused Silica 12 60.17164 7.97588 27.00000 Air 13 −2.6045e+047.50000 30.00000 Fused Silica 14 −107.86582 0.10000 30.00000 Air 15407.32327 8.00000 33.00000 Fused Silica 16 −158.71706 0.1000 33.00000Air 17 41.13105 22.90344 36.50000 Fused Silica 18 476.74020 8.0235136.00000 Air 19 −108.54992 17.81428 36.00000 Fused Silica 20 52.2870832.71346 30.00000 Air 21 744.72666 17.00000 31.00000 Fused Silica 22−47.71508 21.21160 31.00000 Air 23 112.01284 7.00000 21.00000 FusedSilica 24 −132.26649 0.10000 21.00000 Air 25 46.50995 6.00000 18.00000Fused Silica 26 213.25215 13.00000 18.00000 Air 27 — 51.58643 0.13054Air 28 — 26.79611 73.00000 Fused Silica 29 — −26.79611 73.00000 Reflect.30 — −51.58643 73.00000 Air 31 88.23207 51.58643 77.00000 Reflect. 32 —26.79611 73.00000 Fused Silica 33 — 0.75000 73.00000 Air 34 — — 1.00123image

Transferring the pupil to an external plane can affords simultaneousoperation of various dark field schemes. For example, double dark fieldand directional dark field can be performed using the system illustratedin FIG. 34. The system illustrated in FIG. 34 is based on thecatadioptric objective with a relayed pupil. From FIG. 34, thecatadioptric system 3401 employs a laser for oblique illumination asshown by laser beam 3402. Scattered, diffracted, and reflected lightcollected from the object is filtered at the pupil plane by a filter3403 similar to a composite of the filters illustrated in FIGS. 21 and22. The light beam 3406 passing through the central opening in the pupilfilter 3403 corresponds to the directional dark field scheme, whilelight passing through the outer openings of pupil filter 1903corresponds to the double dark field scheme. Light energy for these twodifferent dark field schemes may be separated using a plate 3404 havinga reflective central spot 3406 and directed to separate detectors. Othersimultaneous schemes are also possible, such as normal incidence darkfield imaging using oblique and near normal collection.

Other methods of pupil shaping may be employed. For example, pupilshaping may create simultaneous operation with different dark fieldschemes. A diffractive optic, segmented optic, or other device can beplaced at or near the pupil plane to direct different portions of thepupil to different locations. Multiple detectors or a single scanneddetector can be used.

In a further aspect, a tube lens group can be used with the 0.97, 0.98,and 0.99 NA objectives in systems having a 1.0 mm field size. The tubelens group has the same 30 mm collimated beam diameter as used in thosedesigns as shown in FIGS. 17, 29, 30 and 31 and is designed for thesingle wavelength of 0.266 micrometers. The tube lens group magnifiesthe 1.0 mm field size of the objectives onto a 36 mm detector. Inaccordance with other aspects disclosed herein, the tube lens group isdesigned to have a distant exterior pupil plane to match the buriedinterior pupil of the catadioptric objectives. A similar tube lens groupcan also be designed to work with objectives that have a larger field ofview or a relayed pupil. The Strehl ratio of the tube lens group designis approximately 0.99 over the field. A more complicated tube lens groupdesign is necessary for use with the larger field size objective due notonly to larger field size but also to the lower magnification if thesame size detector is used.

The tube lens group is illustrated in FIGS. 35 and 36 and includeslenses 3501 and 3502 as well as focal plane 3503. A tube lens grouphaving focal length of 555.6 mm and an NA of 0.027 will have thefollowing dimensions for 0.97, 0.98, and 0.99 NA objectives for awavelength of 0.266 micrometers and a 30 mm beam diameter:

Surface Radius Thickness [element] Aperture (mm) (mm) (mm) RadiusMaterial 0 — 1.0000e+20 3.2400e+18 Air 1 — 46.090490 15.000000 Air(Aperture Stop) 2 [3501] −49.980600 10.00000 19.000000 Fused silica 3[3501] −56.665981 1.000000 20.000000 Air 4 [3502] −217.218335 6.00000021.000000 Fused Silica 5 [3502] −111.297311 599.512125 21.000000 Air 6 —— 18.000054 Air

The resultant image is thus 36 millimeters in length.

Still another aspect of the current system uses a six element varifocaltube lens group for 20× to200× as shown in FIG. 37 and which can beindependently corrected from the main system and which providesmagnification and transposition from the pupil plane. At the edge of the38 millimeter detector, for a 20× magnification, the Strehl ratio for a0.266 micrometer wavelength is 0.955. This value improves over the restof the detector and at magnifications greater than 20×. Worst casedistortion from 20× to 200× is 0.06 percent. The nearest distance from alens to the detector is 149 millimeters, and this distance increases by777 millimeters in extending the magnification from 20× to200×. Themotion of the moving group of four lenses, as shown in FIG. 37, is 160mm. Note that both FIGS. 37 and 38 illustrate the varifocal tube lensgroup at 20× magnification.

This varifocal tube lens group design may be used with an objectivehaving a 15 mm diameter pupil, such as with the relayed pupil designdescribed above. It may also be desirable to use this type of avarifocal tube lens group with an objective that does not have a relayedpupil. This is possible using the concepts disclosed herein, withadjustments that would be apparent to those of ordinary skill in theart.

This design is for use at 0.266 micrometers where the refractive indexof fused silica is 1.49968. For the 20× magnification the system hasfocal length of 153 mm and an NA of 0.049. The dimensions are asfollows:

Aperture Surface Radius Thickness [element] (mm) (mm) (mm) RadiusMaterial 0 — 1.0000e+20 1.2456e+19 Air 1 [3708] — — 7.5000000 FourierPlane 2 [3708] — 152.90980 7.5000000 Air 3 [3707] −1002.31432 7.00000030.000000 Fused Silica 4 [3707] −140.48005 192.59159 30.000000 Air 5[3706] 127.96246 10.000000 38.000000 Fused Silica 6 [3706] 1694.87685423.59936 38.000000 Air 7 [3705] 58.02517 8.0000000 21.000000 FusedSilica 8 [3705] −126.99340 1.00992 21.000000 Air 9 [3704] 34.9895617.94389 19.000000 Fused Silica 10 [3704] 136.51285 2.28230 14.500000Air 11 [3703] −132.38435 3.12463 14.000000 Fused Silica 12 [3703]20.90456 5.07104 10.500000 Air 13 [3702] −25.78032 10.53393 10.500000Fused Silica 14 [3702] −27.58716 148.87138 11.500000 Air 15 — — 19.05723

FIG. 39 shows an alternate catadioptric imaging system suited for use inbroadband deep ultraviolet applications and made up of a focusing lensgroup 11 for forming an intermediate image 13, a field lens group 15disposed proximate to the intermediate image 13 for correcting chromaticaberrations, and a catadioptric group 17 for focusing light from theintermediate image 13 to a final image 19. The imaging system has theability to correct both monochromatic (Seidel) aberrations and chromaticaberrations (longitudinal and lateral), as well as chromatic variationsof the monochromatic aberrations, over a wavelength band that extendsinto the deep ultraviolet (UV) portion of the spectrum, covering 0.20 to0.40 micron UV light. The catadioptric system can be adapted for anumber of UV imaging applications, including as a UV microscopeobjective, a collector of surface scattered UV light in a waferinspection apparatus, or as mask projection optics for a UVphotolithography system.

The focusing lens group 11 in FIG. 39 includes seven lens elements21–27, with two of the lens elements (21 and 22) separated by asubstantial distance from the remaining five lens elements (23–27). Theseparations of the pair of lens elements 21 and 22 from the remainingfive lens elements 23–27 is typically on the order of at least one-halfthe total combined thickness of the five lens elements 23–27. Forexample, lens elements 23–27 may span a distance of about 60 millimeters(mm) and lens element 22 may be 30 to 60 mm from lens element 23. Theactual dimensions depend on the scale chosen. The two lenses 21 and 22form a low power doublet for correcting chromatic variation ofmonochromatic image aberrations, such as coma and astigmatism. By havingthis doublet 21 and 22 relatively far from the other system components,the shift of the light beam with field angles on these two lenses ismaximized. That in turn helps greatly in achieving the best correctionof chromatic variation of aberrations.

The five lenses 23–27 of the main focusing subgroup consist of a thickstrong negative meniscus lens 23, an opposite-facing relativelystrongly-curved negative meniscus lens 24, a strong bi-convex lens 25, astrong positive meniscus lens 26, and an opposite-facingstrongly-curved, but relatively very weak, meniscus lens 27 of eitherpositive or negative power. Variations of this lens 23–27 subgroup arepossible. The subgroup focuses the light to an intermediate image 13.The curvature and positions of the lens surfaces are selected tominimize monochromatic aberrations and to cooperate with the doublet21–22 to minimize chromatic variations of those aberrations.

The field lens group 15 typically comprises an achromatic triplet,although any achromatized lens group can be used. Both fused silica andCaF₂ glass materials are used. Other possible deep UV transparentrefractive materials can include MgF₂, SrF₂, LaF₃ and LiF glasses, ormixtures thereof. In addition to refractive materials, diffractivesurfaces can be used to correct chromatic aberrations. Because thedispersions between the two UV transmitting materials, CaF₂ glass andfused silica, are not very different in the deep ultraviolet, theindividual components of the group 15 have strong curvatures. Primarycolor aberrations are corrected mainly by the lens elements in thecatadioptric group 17 in combination with the focusing lens group 11.Achromatization of the field lens group 15 allows residual lateral colorto be completely corrected.

The catadioptric group 17 of FIG. 39 includes a fused silica meniscuslens 39 with a back surface having coating 41, and fused silica lens 43with a back surface having a reflective coating 45. The two lenselements 39 and 43 front surfaces face each other. The reflectivesurface coating 41 and 45 are typically aluminum, possibly with adielectric overcoat to enhance reflectivity.

The first lens 39 has a hole 37 centrally formed therein along theoptical axis of the system. The reflective coating 41 likewise ends atthe central hole 37 leaving a central optical aperture through whichlight can pass unobstructed by either the lens 39 or its reflectivecoating 41. The optical aperture defined by the hole 37 is in thevicinity of the intermediate image plane 13 so that minimum optical lossoccurs. The achromatic field lens group 15 is positioned in or near thehole 37. The second lens 43 does not normally have a hole, but there isa centrally located opening or window 47 where the coating is absent onthe surface reflective coating 45. The optical aperture in lens 39 withits reflective coating 41 need not be defined by a hole 37 in the lens39, but could be defined simply by a window in the coating 41 as incoating 45. In that case, light would pass one additional time throughthe refractive surfaces of lens 39.

Light from the source transmitted along the optical axis toward theintermediate image plane 13 passes through the optical aperture 37 inthe first lens 39 and then through the body of the second lens 43 whereit is reflected by the near planar (or planar) mirror coating 45 backthrough the body of the second lens 43. The light then passes throughthe first lens 39, is reflected by the mirror surface 41 and passes backthrough the first lens 39. Finally the light, now strongly convergentpasses through the body of the second lens 43 for a third time, throughthe optical aperture 47 to the target image plane adjacent aperture 47.The curvatures and positions of the first and second lens surfaces areselected to correct primary axial and lateral color in conduction withthe focal lens group 11.

For a flexible deep UV microscope system, it is important to providevarious magnifications, numerical apertures, field sizes, and colors. Inprinciple, an UV microscope system can comprise several catadioptricobjectives, tube lenses, and zoom lenses. An ultra-broadband UVmicroscope imaging system as illustrated in FIG. 40 comprises acatadioptric objective section 128 and a zooming tube lens groupsections 139. The catadioptric objective section 128 comprises acatadioptric lens group 122, a field lens group 127, and a focusing lensgroup 129. The beam splitter 132 provides an entrance for the UV lightsource. The aperture stop 131 is used to adjust the system imagingnumerical aperture (NA). The microscope system images an object 120(e.g., a wafer being inspected) to the image plane 140.

The catadioptric objective section 128 performs ultra-broadband imagingin the UV spectral region (about 0.20 to 0.40 micron wavelength). It hasexcellent performance for high numerical apertures and large objectfields. This system uses the Schupmann principle in combination with anOffner field lens to correct for axial color and first order lateralcolor, and an achromatized field lens group to correct the higher orderlateral color. The elimination of the residual higher order chromaticaberrations makes the ultra-broadband UV objective design possible.

The catadioptric lens group 122 includes a near planar or planarreflector 123, which is a reflectively coated lens element, a meniscuslens 125, and a concave spherical reflector. Compared to thereflectively coated lens element 39 in FIG. 39, this aspect uses aconcave reflector 124 and a large meniscus lens 125 to simplifymanufacturing. Both reflective elements have central optical apertureswithout reflective material to allow light from the intermediate imageplane 126 to pass through the concave reflector, be reflected by thenear planar (or planar) reflector 123 onto the concave reflector 124,and pass back through the near planar or planar reflector 123,traversing the associated lens element or elements on the way.

The achromatic multi-element field lens group 127 is made from two ormore different refractive materials, such as fused silica and fluorideglass, or diffractive surfaces. The field lens group 127 may beoptically coupled together or alternatively may be spaced slightly apartin air. Because fused silica and fluoride glass do not differsubstantially in dispersion in the deep ultraviolet range, theindividual powers of the several component element of the field lensgroup need to be of high magnitude. Use of such an achromatic field lensallows the complete correction of axial color and lateral color over anultra-broad spectral range. In one aspect of the design, only one fieldlens component need be of a refractive material different than the otherlenses of the system. Compared to group 15 in FIG. 39, the field lensgroup 127 is moved slightly from the intermediate image location toreduce the heat load and surface scattering of the field lens group.

The system may have a focusing lens group 129 with multiple lenselements, preferably all formed from a single type of material, withrefractive surfaces having curvatures and positions selected to correctboth monochromatic aberrations and chromatic variation of aberrationsand focus light to an intermediate image. In the focusing lens group 129a combination of lenses 130 with low power corrects the system forchromatic variation in spherical aberration, coma, and astigmatism.

The zooming tube lens 139 combined with the catadioptric objective 128provides many desirable features. Such an all-refractive zooming lensideally will allow the detector array 140 to be stationary duringzooming, although the invention is not limited to this aspect. Assumingthat the catadioptric objective system 128 does not also have anyzooming function, there are two design possibilities open to the zoomingtube lens system 139.

First, the zooming section 139 can be formed of the same refractivematerial, such as fused silica, and it must be designed so that primarylongitudinal and primary lateral color do not change during zooming.These primary chromatic aberrations do not have to be corrected to zero,and cannot be if only one glass type is used, but they have to bestationary, which is possible. Then the design of the catadioptricobjective 128 must be modified to compensate for these uncorrected butstationary chromatic aberrations of the zooming tube lens. Despite thelimited image quality, this design possibility is very desirable sincethe whole combined microscope system may in certain circumstances beformed of a single material, i.e., fused silica, except for the calciumfluoride or a diffractive surface in the achromatized Offner-type fieldlens.

Second, the zooming tube lens group 139 can be corrected for aberrationsindependently of the catadioptric objective 128. This requires the useof at least two refractive materials with different dispersions, such asfused silica and calcium fluoride, or diffractive surfaces. The resultmay be a tube lens system that, because of unavoidable higher-orderresiduals of longitudinal and lateral color over the entire zoom range,is not capable of high performance over a very broad UV spectral region.Compromises must then be made in the form of reducing the spectralrange, the numerical aperture, the field size of the combined system, orsome combination of these compromises. The result is that the very highcapabilities of the catadioptric objective cannot be duplicated with anindependently corrected zooming tube lens.

The present system straddles the two situations just described. Thezooming tube lens 139 is first corrected independently of thecatadioptric objective 128, using two refractive materials (such asfused silica and calcium fluoride). Lens 139 is then combined with thecatadioptric objective 128 and then the catadioptric objective ismodified to compensate for the residual higher-order chromaticaberrations of the zooming tube lens system. This is possible because ofthe design features of the field lens group 127 and the low power lensgroup 130 of the catadioptric objective described earlier. The combinedsystem is then optimized with all parameters being varied to achieve thebest performance.

One unique feature of the present system is the particular details ofthe zooming tube lens. If the higher-order residual chromaticaberrations of this zooming system change during zoom, then thecatadioptric objective cannot exactly compensate for them except at onezoom position. It is relatively easy for one skilled in the art todesign a zooming tube lens system where the low-order chromaticaberrations do not change during zoom, and are corrected to zero aswell. It can be very difficult to find a zooming tube lens design wherethe higher-order chromatic aberration residuals (which are uncorrectableto zero, in that system by itself) do not change during the zooming.

A tube lens section can be designed such that its higher-order chromaticaberrations do not change by any significant amount during zoom. If thedetector array 140 is allowed to move during zoom, then the designproblem becomes much easier, but that is not nearly as desirable ashaving an image position fixed relative to the rest of the system.

The imaging system of the system provides a zoom from 36× to 100× andgreater, and integrates objectives, turret, tube lenses (to provide moremagnifications) and zoom optics into one module. The imaging systemreduces optical and mechanical components, improves manufacturabilityand reduces production costs. The imaging system has several performanceadvantages such as: high optical resolution due to deep UV imaging,reduced thin film interference effects due to ultra-broadband light, andincreased light brightness due to integration of ultra-broad spectralrange. The wide range zoom provides continuous magnification change. Thefine zoom reduces aliasing and allows electronic image processing, suchas cell-to-cell subtraction for a repeating image array. By placing anadjustable aperture in the aperture stop of the microscope system onecan adjust the NA and obtain the desired optical resolution and depth offocus. The system is flexible with an adjustable wavelength, anadjustable bandwidth, an adjustable magnification, and an adjustablenumerical aperture.

Three possible aspects of zoom lenses are provided. The first aspectprovides linear zoom motion with a fixed detector array position. Thesecond aspect provides linear zoom motion with a moving detector arrayposition. The third aspect, in addition to zoom lenses, utilizes foldingmirrors to reduce the physical length of the imaging system and fix thedetector array position.

The first zoom lenses provide linear zoom motion with a fixed detectorarray position. FIG. 41 shows the 36× zoom arrangement of the lenses,the 64× zoom arrangement of the lenses and the 100× zoom arrangement ofthe lenses. The detector array 140 (not shown) is fixed. The zoomingtube lens design 141 is comprised of two moving lens groups 142 and 143.The beam splitter is not shown in this and later figures for the purposeof clarity. The following table lists the surfaces shown in FIG. 41,where the surface numbering begins at “0” for the final image countingtowards the object being inspected.

Lens Data for the First Aspect

0.90 N.A., fixed detector, 36×–100× zoom, 1.0 mm field size

Surface Radius Thickness Material 0 — 30.000000 36× Air 152.396279 64×318.839746 100× 1 −46.843442 4.000000 Calcium fluoride 2 67.0173790.999804 Air 3 122.003494 7.000000 Silica 4 −34.944144 4.496612 Air 5−42.883889 4.000000 Calcium fluoride 6 −1.5857e+03 339.659737 36× Air298.114540 64× 279.997392 100× 7 −657.423731 9.000000 Calcium fluoride 8−67.124645 0.999689 Air 9 −70.484550 6.000000 Silica 10 102.73201228.382549 Air 11 170.942101 13.000000 Calcium fluoride 12 −126.768482274.177371 36× Air 193.326289 64× 44.999970 100× 13 103.846671 5.000000Silica 14 57.151413 3.500000 Air 15 113.406488 7.000000 Silica 16−149.254887 58.301706 Air 17 41.730749 14.904897 Silica 18 17.37534711.364798 Air 19 −22.828011 5.892666 Silica 20 −57.773872 1.000000 Air21 174.740180 7.000000 Silica 22 −48.056749 4.000000 Air 23 24.02338011.500000 Silica 24 −1.0394e+03 4.198255 Air 25 −43.531092 5.000000Silica 26 −197.030499 1.000000 Air 27 45.618003 29.827305 Silica 28−81.744432 1.662262 Air 29 17.258988 4.000000 Calcium fluoride 30−31.010978 0.315372 Air 31 −24.055515 2.000000 Silica 32 5.6025590.020000 Air 33 5.602559 8.318486 Calcium fluoride 34 −24.8711167.710304 Air 35 — 8.328925 Air Aperture Stop 36 85.000000 11.000000Silica 37 70.542512 29.938531 Air 38 1.6514e+03 10.000000 Silica 39Infinity −10.000000 Reflect 40 1.6514e+03 −29.938531 Air 41 70.542512−11.000000 Silica 42 85.000000 −8.328925 Air 43 74.648515 8.328925Reflect 44 85.000000 11.000000 Silica 45 70.542512 29.938531 Air 461.6514e+03 10.000000 Silica 47 Infinity 1.500000 Air

The second aspect of zoom lenses provides linear zoom motion with amoving detector array position and FIG. 42 shows the 36× zoomarrangement of the lenses, the 64× zoom arrangement of the lenses andthe 100× zoom arrangement of the lenses. The following table lists thesurfaces shown in FIG. 42, where the surface numbering begins at “0” forthe final image incrementing by 1 towards the object being inspected.

Lens Data for the Second Aspect

0.90 N.A., moving detector, 36× to 100× zoom, 1.0 mm field size

Surface Radius Thickness Material 0 Infinity 110.004950 36× Air405.371660 64× 785.131189 100× 1 73.156621 5.000000 Calcium fluoride 2−609.638437 18.230155 Air 3 −30.303090 3.500000 Calcium fluoride 444.361656 4.000000 Air 5 −51.318999 7.765282 Silica 6 −23.2311951.564401 Air 7 −119.756315 4.000000 Calcium fluoride 8 40.00270112.019418 Air 9 54.594789 10.000000 Calcium flouride 10 −28.9237440.100000 Air 11 −29.957411 5.000000 Silica 12 −156.281481 202.434836 36× Air 108.230318 64× 64.650627 100× 13 188.664770 4.500000 Silica 1456.034008 3.500000 Air 15 214.395300 6.000000 Silica 16 −79.84217462.685096 Air 17 29.721624 10.000000 Silica 18 18.529920 11.406390 Air19 −23.406055 5.864347 Silica 20 −46.076628 1.000000 Air 21 94.3109697.000000 Silica 22 −75.041727 4.000000 Air 23 23.509091 11.500000 SilicaAperture Stop 24 −399.710365 4.516455 Air 25 −42.987793 10.000000 Silica26 −217.407455 12.083912 Air 27 24.940148 10.000000 Calcium flouride 28−177.604306 0.100000 Air 29 24.508018 10.000000 Calcium flouride 30−54.909641 0.664880 Air 31 −16.389836 2.000000 Silica 32 4.2968360.020000 Air 33 4.296836 3.000000 Calcium fluoride 34 −14.0142647.000000 Air 35 — 11.160093 — Internal image 36 102.631452 11.000000Silica 37 84.741293 27.845569 Air 38 1.1470e+03 10.000000 Silica 39Infinity −10.000000 Reflect 40 1.1470e+03 −27.845569 Air 41 84.741293−11.000000 Silica 42 102.631452 −11.160093 Air 43 75.033466 11.160093Reflect 44 102.631452 11.000000 Silica 45 84.741293 27.845569 Air 461.1470e+03 10.000000 Silica 47 Infinity 1.500000 Air

A third aspect of zoom lenses provides linear zoom motion with a fixedsensor position by using the same lens design as the second aspect andincorporating a “trombone” system of reflective elements so that thedetector array does not move. FIG. 43 shows the 36× zoom arrangement ofthe lenses and reflective elements, the 64× zoom arrangement of thelenses and reflective elements and the 100× zoom arrangement of thelenses and reflective elements. The folding mirror group 144 is the“trombone” system of reflective elements. This folding mirrorarrangement is just one example. Many other arrangements are possible,such as, using a different number of reflective elements.

Module Transfer Function curves (not shown) indicate that the FIG. 43aspect is essentially perfect at 64× and 100×, and is good at 36×.Zooming is done by moving a group of six lenses, as a unit, and alsomoving the arm of the trombone slide. Since the trombone motion onlyaffects focus and the f-speed at location is very slow, the accuracy ofthis motion could be very loose. One advantage of the trombone aspect isthat it significantly shortens the system. Another advantage is thatthere is only one zoom motion that involves active (non-flat) opticalelements. And the other zoom motion, with the trombone slide, isinsensitive to errors.

FIG. 15 is a schematic side view of a catadioptric imaging system with azoom in an application for the inspection of semiconductor wafers.Platform 80 holds a wafer 82 that is composed of several integratedcircuit dice 84. The catadioptric objective 86 transfers the light raybundle 88 to the zooming tube lens 90 which produces an adjustable imagereceived by the detector 92. The detector 92 converts the image tobinary coded data and transfers the data over cable 94 to data processor96.

A further aspect of the present design is that of operating in deepultraviolet or vacuum ultraviolet conditions. In one aspect of the deepultraviolet/vacuum ultraviolet design, a method of bright field or ringdark field inspection, and is illustrated in FIG. 16. This method isparticularly suited for photomask or wafer inspection and comprisesillumination optics such as transmission illumination source 1601 orreflected illumination source 1605, a long working distance catadioptricimaging objective 1602, image forming optics 1603, and a detector 1604.For wafer inspection, only reflected light illumination would berequired. The design of the long working distance catadioptric objectiveand the image forming optics are enabling technologies for photomaskinspection at wavelengths at or below 365 nm. The optics and detectorare all carefully designed and optimized for the wavelength and spectralbandwidth of the illumination.

Different types of illumination sources may be employed in this designand incorporated in the transmitted light illumination 1601 andreflected light illumination 1605 and with the designs of aspects 3–10and with respect to FIGS. 47–59. These light sources include but are notlimited to excimer lasers and lamps. Different wavelength lasers arepossible using frequency mixing techniques. Many different lamp sourcesare available including mercury xenon (365–220 nm), cadmium lamp(210–220 nm), deuterium (150–190 nm), as well as various excimer lamps.These sources can have very different spectral bandwidths. For example,an unnarrowed excimer laser, a lamp with a bandpass filter, or afrequency converted laser can each produce light having a 1 nm bandwidthor less. An unfiltered lamp or lamp with a larger bandpass filter, suchas an arc lamp, excimer lamp, or a deuterium lamp, are also possiblesources having bandwidth greater than 1 nm. Relatively few light sourcesare available at 193 nm and 157 nm wavelengths. Of these light sources,excimer lasers have the brightness required to support high speedphotomask inspection. The illumination used in this aspect can be eithertransmitted light, shown as transmitted illumination source 1601,reflected light, shown as reflected illumination source 1605, or both.As noted, the illumination may also be in the form of a ring such asrequired for ring dark field imaging. For example, this ringillumination can be obtained by placing a ring shaped aperture at ornear a pupil plane located in the illumination system. This aperture mayblock small illumination angles near the optical axis of the pupil andallow higher illumination angles away from the optical axis of thepupil. This ring should be matched to a similar ring in the catadioptricobjective or image forming optics such that all the rays from theillumination ring are blocked in the image.

Note that in the case of the reflected illumination source thatbeamsplitter/reflector 1606 is employed. In the case of transmittedillumination using transmitted illumination source 1601, photomask 1607is disposed between transmitted illumination source 1601 and objective1602. The illumination system for transmitted light may preferablyemploy a condenser objective. The purpose of the condenser objective isto illuminate a region nominally the same size as the imaging region ona wafer or photomask. As the condenser objective is used only forillumination, it does not require high optical quality. Such anobjective can be a reduced performance version of the catadioptricobjectives presented in this system or a simple all refractive design.Such designs are possible for those skilled in the art when presentedwith this disclosure. The illumination for reflected light uses abeamsplitter and is implemented as in a standard microscope.

The long working distance imaging objective 1602 includes thosedescribed in designs 3 through 8 and illustrated in FIGS. 47–57. Theobjective working distance may be greater than 6 mm so it will notinterfere with a pellicle that protects the photomask. The objective mayalso be well corrected for aberrations over the bandwidth of theillumination source. Many of the available illumination sources have abandwidth that is greater than the 1–2 pm bandwidth obtained from astandard type of single material all refractive objective design. Anexample of this is an unnarowwed excimer laser that typically has aspectral bandwidth around 1 nm. The catadioptric designs 3–8 addressthis problem. The objective may also be capable of imaging over a largefield. Large fields and high data acquisition rates are essential tomake inspecting the photomask as fast as possible. There are similarrequirements for wafer inspection, however a shorter working distance ispossible.

The image forming optics 1603 may be corrected over the spectralbandwidth of the illumination source and the catadioptric imagingobjective. The image forming optics 1603 should also be capable ofvarious magnifications required in a photomask inspection environment.The image forming optics 1603 and the catadioptric objective 1602 mayeach and together be fully corrected for aberrations. Such aberrationcorrection permits testing the image forming optics 1603 and thecatadioptric objective 1602 as separate units. Alternately, aberrationcorrection may be shared between the catadioptric objective 1602 and theimage forming optics 1603.

The image forming optics or catadioptric objective may also contain aring aperture at or near a pupil plane such as required for ring darkfield imaging. This aperture may block all direct illumination lightfrom reaching the detector, corresponding to a similar aperture in theillumination. Thus the illumination aperture may be a transmissive ringand the imaging aperture may be a corresponding opaque ring.Combinations of bright field imaging and ring dark field imaging arealso possible. They may be combined on separate detectors or on the samedetector at the same times or in an alternate fashion. When on the samedetector at the same time, the relative intensity and phase can bemodified by suitable apertures in the illumination and imaging.

In the DUV-VUV aspect of the current design, the detector 1604 ispreferably a high speed detector capable of the high data rates used forinspection systems. Detector 1604 can be a single point diode typedetector or an area type detector such as a CCD or a CCD operating inthe Time Delay and Integration (TDI) mode. This detector 1604 may have ahigh quantum efficiency, low noise, and a good Modulation TransferFunction (MTF). Back thinned CCD sensors can be used for this purpose.

The second aspect is a method for laser dark-field inspection at orbelow 365 nm, and is illustrated in FIG. 46. It is suited for wafer andphotomask inspection. This apparatus consists of illumination optics,such as laser illumination element 4601, a long working distancecatadioptric imaging objective 4602, a Fourier filter or aperture 4603at the external pupil plane, image forming optics 4604, and a detector4605. Catadioptric imaging designs using two glass materials areachievable when using an excimer laser illumination source with greaterthan 1 nm bandwidth.

The types of illumination that can be used for this system are similarto those used for bright field and ring dark field inspection. Onemethod or apparatus for laser-dark field illumination of a semiconductorspecimen is direct illumination of the specimen from outside theobjective. In such an arrangement, only light scattered from thespecimen is collected by the catadioptric objective. The specularlyreflected beam is beyond the numerical aperture of the objective and isnot collected.

Again, the long working distance imaging objective 4602 is describedwith respect to FIGS. 47–57. For laser dark field inspection, thesedesigns should fulfill certain basic requirements. The long workingdistance offered by the designs of FIGS. 47–57 simplifies deliveringlaser energy to the wafer in the semiconductor specimen environment fromoutside the objective without interfering with the operation of theimaging system. The objective is preferably also well corrected foraberrations over the bandwidth of the illumination source 4601. Many ofthe available illumination sources have a bandwidth that is greater thanthe 1–2 pm bandwidth obtained from a standard type of single materialall refractive objective design. The objective 4602 images over a largefield, as large fields and high data acquisition rates provide for rapidwafer or photomask inspection.

The objective also preferably has an easily accessible pupil plane tosupport Fourier filtering or aperturing, such as by the Fourier filteror aperture 4603. Fourier filtering can reduce the noise caused byrepeating patterns on the wafer, thereby permitting smaller randomdefects to be more readily detected.

The image forming optics 4604 may be corrected over the spectralbandwidth of the illumination and the catadioptric imaging objective4602. The image forming optics 4604 also preferably support the variousmagnifications required by a dark field inspection system. Oneimplementation of the image forming optics 4604 is to have them and thecatadioptric objective 4602 each fully corrected for aberrations. Such asystem allows simplified testing of image forming optics 4604 and thecatadioptric objective 4602 as separate units. An alternate technique issharing aberration correction between the catadioptric objective 4602and the image forming optics 4604. Such an approach can be mechanicallyor optically simpler, but can complicate image forming optics andcatadioptric objective testing.

Again, as shown in FIG. 16, the detector 4605 is preferably a high speeddetector capable of the high data rates used for an inspection system.Detector 4605 can be a single point diode type detector or an area typedetector such as a CCD or a CCD operating in the Time Delay andIntegration (TDI) mode. Ideally, this detector should have a highquantum efficiency, low noise, and a good Modulation Transfer Function(MTF). Such detectors are generally known to by those skilled in theart.

The design of FIGS. 16 and 46 can support bright field, laserdirectional dark field, ring dark field, and simultaneous bright-fieldand dark-field schemes, where each of these schemes can be achieved inthe presence of UV, DUV, and VUV wavelengths. The opaqueness of CMPlayers in the deep UV and VUV ranges makes a system using this objectiveideally suited to finding surface defects and microscratches onsemiconductor wafers.

The catadioptric optical apparatus presented to support bright field anddark field imaging and inspection are also ideal for use in a variety ofother applications. The design can be easily optimized by one skilled inthe art for wavelengths from the visible range to the deep UV range andto the vacuum UV range. Longer wavelengths can be optimized for largerbandwidths because the glass dispersion is less. For example, bandwidthsof greater than 140 nm are possible with a two material design and acenter wavelength of 300 nm. The light energy can include shorterwavelengths and the design permits use of multiple wavelengths. Forsemiconductor inspection, the designs presented can support brightfield, laser directional dark field, ring dark field, and simultaneousbright-field and dark-field schemes. The optical designs presented arealso suited for use as a lithography lens or for lithography simulation,a research tool for micro-electronic development, florescencemeasurements, or in biology where a long working distance is requiredfor sample clearance. Due to the ability of this objective to provideapplications in the presence of extremely varied light wavelengths andspectral bandwidths, the designs in FIGS. 16 and 46 are well suited forflorescence measurements.

FIG. 47 illustrates a third aspect of the design. This third aspect issimple example of a folded 0.7 NA catadioptric objective 300 utilizing asingle glass material. This example is presented mainly to simplify theexplanation of the aberration correction and objective functionality.FIG. 47 includes a line in the upper left corner indicating a referencemeasurement of the number of millimeters the line represents. Also, asin other figures discussed herein, light is shown entering from the leftside of FIG. 47 from an excimer laser energy source 47000 (not shown).The energy from the energy source 47000 is focused by a group ofpositive and negative lenses 4701 arranged in either a telephoto orinverse telephoto configuration. The positive lens may be either in thefront or in the back of the arrangement, where front and back arerelative terms with respect to the energy source. Here negative lens4702 is in front of positive lens 4703, where negative lens 4702 andpositive lens 4703 form the group of positive and negative lenses 4701.A positive field lens 4704 is located somewhat in front of the focus forlens group 4701. Very close to the small image formed is located arelatively small pair of flat fold mirrors 4705 arranged in a V-shape.The first small flat mirror 4705 a reflects the light at 90 degrees tothe optical axis. Some other angular amount may be used depending on theapplication desired while still within the course and scope of thepresent system. The diverging light then proceeds to the Mangin mirror4706 located at the bottom of FIG. 47. A Mangin mirror is a lens/mirrorelement that is refractive and has a reflective back surface. The Manginmirror 4706 re-images the first image onto the second small flat foldingmirror 4705 b, at substantially unit magnification. The system isimplemented far enough off axis such that the returning second image isdisplaced laterally enough from the first image so that the lateralseparation permits optical manipulation of each separate image. Theamount of lateral separation allows the second small flat mirror 4705 bto fold the light path back onto the original optical axis. This effectis illustrated in FIG. 48.

According to FIG. 48, the input rays in the field of the substantiallyunit magnification Mangin mirror relay are on one side of the field andthe output rays are on the opposite side. From FIG. 47, the second smallfold mirror 4705 b is followed by a second field lens 4707. From therethe light proceeds up to the final focusing lenses 4708 discussed below.This final focusing lens group 4708 provides a relatively long workingdistance to the surface of the specimen, such as the photomask orsemiconductor wafer.

If the two field lenses were not present in the design of FIG. 47, thisdesign would constitute an application of the Schupmann type designusing a Mangin mirror. The Mangin mirror would provide the means bywhich the virtual image of the Schupmann system would be turned into areal image, just as shown in previously known systems. This newconfiguration provides two significant advantages over the previousdesigns: lack of obscuration and a relatively long working distance.Both advantages result from the novel arrangement of the small foldmirrors 4705, the substantially unit magnification Mangin mirror 4706,and the presence of the two intermediate images in the system.

The small fold mirrors 4705 can be implemented in a variety of ways.Objective designs can be optimized where these fold mirrors are on thesides of the field lenses opposite to the internal images. Objectivedesigns can also be optimized where the small fold mirrors are on theside of the internal image closest to the Mangin mirror. Also, a prismor prisms can be used for the reflective surfaces in a variety of ways.A reflective coating can be added to one or more of the prism surfaces.Two prisms can then be used as reflective mirrors. Alternately, oneprism can have two surfaces coated and serve as both reflectivesurfaces. This is advantageous because a highly accurate angle can bepolished on the prism to define the angle of the optical axis. Theprisms can also be used in total internal reflection mode. This isadvantageous when high efficiency optical coatings are difficult, suchas for short wavelengths or broad spectral bandwidths. In this mode, thehypotenuse of a near 90 degree prism is used in total internalreflection, replacing a reflecting surface. The surfaces of the prismthat are near normal to the incoming and reflecting beams may be antireflection coated to improve transmission efficiency.

Addition of one field lens to the system, in either of the two locationswhere the design has its two field lenses, provides the ability tocorrect the design for either secondary axial color or primary lateralcolor. Primary axial color is corrected without field lenses bybalancing the positive refractive power of the Mangin mirror element4706 with the positive power of the lenses, as in connection with theSchumpmann principle. Use of two field lenses 4704 and 4707 near theintermediate images provides for correction of both secondary axialcolor and primary lateral color. The result is a design with a fairlybroad spectral range having good axial color correction, but one that islimited by secondary axial color. Correction of the secondary lateralcolor can be accomplished by balancing the secondary color between thefirst half of the system with the second half of the system, where thefirst half of the system includes all lenses from the incoming energysource up to the reflective portion of the Mangin mirror 4706, includinglens group 4701, field lens 4704, first small fold mirror 4705 a, andMangin mirror 4706. This secondary lateral color balancing scheme workswell due to the two intermediate images produced as in FIG. 48 and thetwo separate field lenses 4704 and 4707 present in the system. As inknown systems, tertiary axial color is improved by moving the fieldlenses 4704 and 4707 a significant distance to one side of theintermediate images.

The present design is a catadioptric system requiring a singlerefractive material, in conjunction with the particularly describedarrangement of mirrors, to correct for chromatic aberrations. For adesign intended for use in the presence of an excimer laser sourcewavelength near 193 nm, the preferred lens material is silica. For asimilar design intended for use in wavelengths near 157 nm, thepreferred lens material is CaF₂. At 157 nm wavelength, for example, CaF₂is preferred since it does not have severe problems with birefringence,water solubility, or mechanical softness. Further chromatic correctioncan also be achieved using two glass materials, but such an arrangementmay require additional cost or present birefringence, water solubility,or mechanical softness drawbacks.

Special challenges are presented when correcting various coloraberrations when only one glass type is used. Conventional designsusually use two or three glass types to correct color aberrations. Thepresent invention performs the correction in the presence of a singlematerial type used in all lenses due to the specific lens and mirrorconfiguration. In very deep UV, both silica and CaF₂ are highlydispersive, so even a narrow spectral bandwidth at very shortwavelengths can require the correction of quite a few distinct coloraberrations. Such color aberrations may include primary and secondaryaxial color, primary and secondary lateral color, chromatic variation ofspherical aberration, and chromatic variation of coma. In the presentsystem, lens and mirror positioning permits primary axial and lateralcolor to be completely corrected. Secondary axial and lateral colorcannot be completely corrected, but can be kept small enough to beacceptable over a relatively narrow spectral bandwidth. Chromaticvariation of both spherical aberration and coma can also be correctedusing this small fold mirror and dual field lens design. The physicalseparation between positive and negative axial color contributionspresent in the design of FIG. 47, particularly the separation topositionally disparate locations within the design, leads to specialproblems in correcting the chromatic variation of aberrations.Minimization of chromatic aberration variations requires a veryparticular arrangement of lens powers and shapes such as illustrated inFIG. 47 and later in designs 4–8.

The design of FIG. 47 provides an optical system having a long workingdistance between the optical system and the surface being inspected,particularly between the final lens in the system 4708 and the surfaceof the object or specimen 4709 (not shown) being imaged. The arrangementof FIG. 47 further provides a high numerical aperture and no centralobscuration. A high numerical aperture provides for high resolutionimaging and collecting as large an angular range above the surface beingimaged as possible. Numerical apertures of greater than 0.8 can beachieved in the DUV-VUV inspection design with excellent performance. Anumerical aperture of 0.8 corresponds to collecting angles above thesurface from normal to 53 degrees. Further, unlike many catadioptricoptical systems, the design of FIG. 47 has no central obscuration toblock low frequency information. The FIG. 47 design does not have thisproblem and permits utilization of all low frequency information.

The design illustrated in FIG. 47 also provides relatively reasonabletolerances which can be more easily manufactured. The benefit ofreasonable tolerances in the design of FIG. 47 is that it overcomesproblems present in many known high NA, broad bandwidth systems havingsome optical elements with very tight position and thickness tolerances.These tight tolerances may make previous designs either too expensive orin some cases impossible to build and operate in a productionenvironment.

FIG. 49 shows a more complex version of the design that providesadditional narrow band aberration correction to give a 0.7 NA, longworking distance, unobscured design using only fused silica. The objectextends from 0.25 mm to 0.75 mm off axis and the design has a bandwidthof 1 nm from 192.8–193.8 nm. The surface data for an example of thistype of design is listed in Table 1. Performance for the system shown inFIG. 49 is limited by chromatic variation in aberrations rather thanhigher order axial color. Other versions are possible and would requireminimal effort by those of ordinary skill when presented with thisdisclosure.

As illustrated in FIG. 49, energy, such as laser energy, is transmittedfrom energy source 49000 (not shown) and into objective 4900. Objective4900 includes lens arrangement 530, which includes a first lens pair 531including first lens 4901 and second lens 4902, followed by third lens4903, fourth lens 4904, and fifth lens 4905. Energy is focused by lensarrangement 4930 toward field lens 4906, which then directs energytoward the small folding mirror or reflecting surface, here specificallyfirst mirror 4907. Energy is directed from first mirror 4907 toward lens4908 and to Mangin mirror 4909, which reflects light energy back throughlens 4908 and toward the second part of the small folding mirror orreflecting surface, specifically to second mirror 510.

In FIG. 49, light is reflected from second folding mirror 4910 to fieldlens 4933, which includes first field lens 4911 and second field lens4912. From the field lens 4933, light is transmitted to focusing lensarrangement 4934, which includes first focusing lens 4913, secondfocusing lens 4914, third focusing lens 4915, fourth focusing lens 4916,fifth focusing lens 4917, sixth focusing lens 4918, and seventh focusinglens 4919. The specimen or surface 520 to be examined is not shown inFIG. 49, but is located to the right of the objective 4900 in theorientation of FIG. 49. Light energy strikes the specimen and reflectsback through the objective 4900 of FIG. 49. Alternately, light energycan transmit through the specimen and then through the objective 4900from right to left as shown in FIG. 49.

TABLE 1 Surface data for the folded design of FIG. 5 operating at 193 nmwavelength with a 1 nm bandwidth Element Surf Radius Thickness* MaterialNumber OBJ Infinity Infinity N/S** STO Infinity −32.038 N/S 2 81.9835.000 Silica 4901 3 18.305 4.216 4901 4 −27.049 5.000 Silica 4902 5−61.738 33.261 4902 6 1276.054 7.000 Silica 4903 7 −83.831 0.500 4903 879.482 8.000 Silica 4904 9 −229.250 1.000 4904 10 47.191 7.000 Silica4905 11 138.080 66.471 4905 12 14.544 4.000 Silica 4906 13 514.189 7.6974906 14 Infinity 0.000 Mirror 4907 15 Infinity −77.947 4907 16 50.786−8.000 Silica 4908 17 139.802 −22.299 4908 18 32.934 −8.000 Silica 490919 60.774 8.000 Mirror 4909 20 32.934 22.299 4909 21 139.802 8.000Silica 4908 22 50.786 77.947 4908 23 Infinity 0.000 Mirror 4910 24Infinity −3.500 4910 25 Infinity −2.500 4910 26 −50.929 −5.724 Silica4911 27 −41.159 −1.814 4911 28 302.166 −6.000 Silica 4912 29 24.557−48.713 4912 30 544.674 −6.500 Silica 4913 31 89.379 −30.822 4913 32−90.727 −12.569 Silica 4914 33 −64.505 −6.000 4914 34 −233.637 −8.000Silica 4915 35 83.534 −0.500 4915 36 −92.578 −5.679 Silica 4916 371208.052 −0.500 4916 38 −50.386 −5.8316 Silica 4917 39 −146.956 −0.5004917 40 −30.274 −5.971 Silica 4918 41 −56.351 −0.500 4918 42 −12.744−7.155 Silica 4919 43 −12.195 −14.453 4919 44 Infinity 1.68E−05 N/S IMAInfinity N/S *Surface thickness represents the thickness of a surfacewhen at the “upstream” side of the element or distance between thesurface and the next surface if at the “downstream” side of the element.For example, surface 2 on element 4901 has a radius of 81.983millimeters and the lens is 5.000 millimeters thick. Surface 3 of lenselement 4901 has a radius of 18.305 millimeters and is 4.216 millimetersfrom the next surface, which is surface 4 on element 4902. **Certainelements relating to but not critical to the design are not shown in thefigures.

A fourth aspect is presented in FIG. 50. FIG. 50 illustrates a folded0.7 NA catadioptric objective using two materials. This design usessilica and calcium fluoride to further increase system bandwidth.Calcium fluoride is added to the field lenses in this design make bothsuch lenses achromats. The design of FIG. 50 is corrected for lightenergy from 193 to 225 nm. Surface data for a system of the fourthaspect is presented in Table 2.

As shown in FIG. 50, light energy or laser energy is transmitted fromenergy source 50000 (not shown) and into objective 5000. Objective 5000includes a lens arrangement 5030, which includes first lens 5001, secondlens 5002, third lens 5003, fourth lens 5004, fifth lens 5005, and sixthlens 5006. Energy is focused by lens arrangement 5030 toward field lensarrangement 5031, which includes seventh lens 5007, eighth lens 5008,ninth lens 5009, and tenth lens 5010. Eighth lens 5008 and tenth lens5010 are formed of calcium fluoride (CaF₂). This field lens arrangement5031 directs light energy to first small folding mirror or reflectingsurface 5011, which directs energy toward lens 5012 and to Mangin mirror5013. Light energy reflects back from Mangin mirror 5013 back throughlens 5012 and toward the second part of the small folding mirror orreflecting surface, specifically to second mirror 5014.

Light is reflected from second folding mirror 5014 to second field lensarrangement 5032, which includes first field lens 5015, second fieldlens 5016, and third field lens 5017. Both first field lens 5015 andthird field lens 5017 are formed of calcium fluoride. From the fieldlens arrangement 5032, light energy is transmitted to focusing lensarrangement 5033, which includes first focusing lens 5018, secondfocusing lens 5019, third focusing lens 5020, fourth focusing lens 5021,fifth focusing lens 5022, and sixth focusing lens 5023. The specimen orsurface 5024 to be examined is not shown in FIG. 50, but is located tothe right of the objective 5000 in the orientation of FIG. 50.

TABLE 2 Surface data for a folded design at 193 nm with a 32 nmbandwidth Element Surf Radius Thickness Material Number OBJ InfinityInfinity N/S STO Infinity −32.204 N/S 2 −101.014 3.000 Silica 5001 325.381 74.761 5001 4 −29.809 5.000 Silica 5002 5 −31.309 1.000 5002 6183.355 7.500 Silica 5003 7 −109.361 0.500 5003 8 99.287 7.000 Silica5004 9 −250.439 1.000 5004 10 50.010 5.500 Silica 5005 11 118.693 3.8825005 12 −216.754 4.000 Silica 5006 13 127.307 44.956 5006 14 28.38924.000 Silica 5007 15 7.676 0.683 5007 16 12.387 3.000 CaF₂ 5008 17−10.095 0.475 5008 18 −8.982 5.497 Silica 5009 19 8.000 0.093 5009 208.069 3.000 CaF₂ 5010 21 −9.002 5.673 5010 22 Infinity 0.000 Mirror 501123 Infinity −94.049 5011 24 60.160 −8.000 Silica 5012 25 158.201 −20.4715012 26 34.636 −8.000 Silica 5013 27 65.450 8.000 Mirror 5013 28 34.63620.471 5013 29 158.201 8.000 Silica 5012 30 60.160 94.049 5012 31Infinity 0.000 Mirror 5013 32 Infinity −3.500 5013 33 Infinity −2.5005013 34 −26.228 −2.500 CaF₂ 5014 35 12.608 −0.102 5014 36 12.417 −9.995Silica 5015 37 −34.270 −2.110 5015 38 70.277 −2.000 CaF₂ 5016 39 25.599−36.553 5016 40 −139.138 −7.000 Silica 5017 41 75.025 −63.911 5017 42−347.832 −6.000 Silica 5018 43 −60.817 −6.000 5018 44 −125.144 −9.000Silica 5019 45 146.610 −0.500 5019 46 −69.321 −9.000 Silica 5020 47482.420 −0.500 5020 48 −44.275 −10.217 Silica 5021 49 −474.223 −0.5005021 50 −19.707 −10.437 Silica 5022 51 −30.002 −18.250 5022 52 Infinity3.14E−05 N/S IMA Infinity

It is also possible to use a diffractive optic instead of a second glassmaterial to increase the bandwidth. In this case, the diffractive opticmust be manufactured with a specific phase profile to ensure properdiffraction efficiency and angles. This would be possible for thoseskilled in the art once presented with this disclosure.

The aspects illustrated in FIGS. 49 and 50 have two disadvantages.First, the optical axis of the Mangin mirror image relay is at 90degrees to the optical axis defined by the focusing lenses. Thisarrangement can mandate very high angular and position tolerances forthe optical elements in the Mangin mirror image relay. This can resultin manufacturing difficulties and increased system cost. For thisreason, it is desirable to have a minimum number of lens elements inthis 90 degree path. Thus, the 90 degree optical axis can limit thedesign options for this objective. Second, the pupil plane foraperturing and Fourier filtering is located in a noncollimated regioninside the objective. This can produce problems for introducingapertures and filters. Also, because the pupil is in a noncollimatedregion, it is not at the Fourier plane of the object being inspected.This can significantly reduce the effectiveness of Fourier filtering.

The fifth aspect, illustrated in FIG. 51, solves the problems of the 90degree bend issue with respect to the Mangin mirror image relay and theinternal pupil plane. FIG. 52 illustrates an in-line or straight 0.7 NAcatadioptric objective employing a single glass material. Thearrangement of FIG. 51 also allows for improved design performance andrelaxes manufacturing tolerances. For example, the decentering of anylens element by 5 microns will cause less than one quarter wave of comawithout using any compensation elements. Using element decenters andtilts as compensation elements, the tolerances become even more relaxed.The arrangement of FIG. 51 includes one bend with some lenses after thesecond internal image. These lenses have extremely relaxed tolerancesand tend not to affect the manufacturability of the system. Thearrangement of FIG. 51 also has an external pupil plane 5101 foraperturing and Fourier filtering. This pupil plane is in the collimatedregion so it corresponds to the Fourier plane of the object. The objectin the arrangement of FIG. 51 extends from 0.25 mm to 0.75 mm off axisand the design has a bandwidth of 1 nm from 192.8–193.8 nm.

As shown in FIG. 51, light energy or laser energy is transmitted fromenergy source 51000 (not shown) and into objective 5100. Objective 5100includes first lens 5102 and first field lens 5103. This first fieldlens 5103 directs light energy to small folding mirror or reflectingsurface 5104, which directs energy toward focusing lenses 5105 and 5106and to Mangin mirror 5107. Light energy reflects back from Mangin mirror5107 back through focusing lenses 5106 and 5105 and past small foldingmirror or reflecting surface 5104. Light energy then passes throughsecond field lens 5108 and through focusing lens arrangement 5120, whichincludes first focusing lens 5109, second focusing lens 5110, thirdfocusing lens 5111, fourth focusing lens 5112, fifth focusing lens 5113,sixth focusing lens 5114, and seventh focusing lens 5115. The specimenor surface 5116 to be examined is not shown in FIG. 51, but is locatedto the right of the objective 5100 in the orientation shown in FIG. 51.Light energy strikes the specimen 5117 and reflects back through theobjective 5100 of FIG. 51. Surface data for a system employing thedesign of FIG. 51 is listed in Table 3.

In a darkfield arrangement, light energy is directed toward the specimensurface as shown in FIG. 2. Light energy may scatter toward theobjective, i.e. toward seventh focusing lens 5105 in the design of FIG.51. In such an arrangement, light energy passes back through the system,striking the Mangin mirror 5107 and passing through first lens 5102. Thedesign of FIG. 51 provides an optical system with an external pupilplane 5101 to support aperturing and Fourier filtering. An aperture canbe used in connection with the FIG. 51 design to provide control of thenumerical aperture of the imaging system. Such an aperture could beplaced at the pupil plane hereby permitting control of overallresolution and depth of focus. Fourier filtering is very important forapplications like laser dark field. Fourier filtering permits filteringsurface patterns that repeat by increasing the signal-to-noise ratio fordefects on the surface.

TABLE 3 Surface data for linear design shown in FIG. 51 at 193 nm with a1 nm bandwidth Element Surf Radius Thickness Material Number OBJInfinity Infinity N/S STO Infinity 25.000 5101 2 −222.386 4.000 Silica5102 3 −28.670 40.503 5102 4 39.160 2.500 Silica 5103 5 177.023 10.000Silica 5103 6 Infinity 0.000 Mirror 5104 7 Infinity −98.015 5104 8−320.423 −6.000 Silica 5105 9 149.893 −142.852 5105 10 58.250 −8.000Silica 5106 11 210.014 −20.290 5106 12 41.193 −9.000 Silica 5107 1381.848 9.000 Mirror 5107 14 41.193 20.290 5107 15 210.014 8.000 Silica5106 16 58.250 142.852 5106 17 149.893 6.000 Silica 5105 18 −320.42398.015 5105 19 Infinity 44.416 5105 20 70.917 15.000 Silica 5108 2148.487 83.467 5108 22 326.205 11.000 Silica 5109 23 −86.355 83.991 510924 235.491 9.000 Silica 5110 25 −111.089 10.357 5110 26 −58.901 4.000Silica 5111 27 −3728.698 89.493 5111 28 45.959 6.365 Silica 5112 2941.432 9.332 5112 30 −739.118 6.000 Silica 5113 31 −79.014 1.000 5113 3244.790 9.000 Silica 5114 33 182.972 1.000 5114 34 22.072 20.822 Silica5115 35 36.911 11.529 5115 IMA Infinity N/S

The sixth aspect of the DUV-VUV design presented in FIG. 52 is similarto the fifth aspect of FIG. 51, but has been optimized for a wavelengthof 157 nm. The change in wavelength requires changing the material usedfrom fused silica to calcium fluoride. The index of fused silica at awavelength of 193 nm is nearly identical to the index for calciumfluoride at 157 nm, so the design requires no major changes aside fromthe material. However, the dispersion of calcium fluoride at 157 nm islarger than the dispersion of fused silica at 193 nm, which may requiresome minor changes to further optimize the design. The design presentedin FIG. 52 also provides the option of splitting the Mangin mirror intoa front surface mirror and a meniscus lens. This can simplifymanufacturing in some cases. This approach can be used on the othercatadioptric objective designs of FIGS. 47–52 as well. The objectextends from 0.25 mm to 0.75 mm off axis and the design has a bandwidthof 0.5 nm from 156.75–157.25 nm.

As shown in FIG. 52, light energy or laser energy is transmitted fromenergy source 52000 (not shown) and into objective 5200. This objective5200 also has an external pupil plane 5201 as in the design presented inFIG. 51. Objective 5200 includes first lens 5202 and first field lens5203. This first field lens 5203 directs light energy to small foldingmirror or reflecting surface 5204, which directs energy toward focusinglenses 5205, 5206, and 5207 and to mirror surface 5208. Light energyreflects back from mirror surface 5208 back through focusing lenses5207, 5206, and 5205 and past small folding mirror or reflecting surface5204. Light energy then passes through second field lens group 5220which includes first field lens 5209, second fiend lens 5210, and thirdfield lens 5211. Light then passes through focusing lens arrangement5221, which includes first focusing lens 5212, second focusing lens5213, third focusing lens 5214, fourth focusing lens 5215, fifthfocusing lens 5216. The specimen or surface 5217 to be examined is notshown in FIG. 52, but is located to the right of the objective 5200 inthe orientation shown in FIG. 52. Light energy strikes the specimen 5217and reflects back through the objective 5200 of FIG. 52. Light fromspecimen 5217 can be apertured or Fourier filtered at pupil plane 5201as described in the fifth aspect presented in FIG. 51.

The surface data for a system having the objective shown in FIG. 52 islisted in Table 4.

TABLE 4 Surface data for a linear design as shown in FIG. 52 at 157 nmwith a 0.5 nm bandwidth Element Surf Radius Thickness Material NumberOBJ Infinity Infinity N/S STO Infinity 25.000 5201 2 −228.090 4.000 CaF₂5202 3 −26.271 34.743 5202 4 27.340 2.500 CaF₂ 5203 5 78.838 10.000 52036 Infinity 0 Mirror 5204 7 Infinity −75.368 5204 8 332.074 −6.000 CaF₂5205 9 75.615 −84.484 5205 10 53.058 −7.000 CaF₂ 5206 11 129.105 −12.0005206 12 40.981 −7.000 CaF₂ 5207 13 128.091 −7.480 5207 14 69.614 7.480Mirror 5208 15 128.091 7.000 CaF₂ 5207 16 40.981 12.000 5207 17 129.1057.000 CaF₂ 5206 18 53.058 84.484 5206 19 75.615 6.000 CaF₂ 5205 20332.074 75.368 5205 21 Infinity 39.173 22 24.018 15.000 CaF₂ 5209 2326.501 24.354 5209 24 −15.238 13.419 CaF₂ 5210 25 −26.901 1.000 5210 26355.973 7.000 CaF₂ 5211 27 −56.508 110.232 5211 28 58.858 10.000 CaF₂5212 29 1338.307 6.0469 5212 30 −54.890 10.000 CaF₂ 5213 31 −72.55626.319 5213 32 −308.917 6.500 CaF₂ 5214 33 −95.467 1.000 5214 34 34.07948.000 CaF₂ 5215 35 110.300 1.000 5215 36 16.407 9.697 CaF₂ 5216 3729.683 11.927 5216 IMA Infinity

The seventh aspect illustrated in FIG. 53 has similarities to the fifthand sixth aspects presented in FIGS. 51 and 52. The design presented inFIG. 53 uses the straight through 0.7 NA catadioptric approach to allowmore design flexibility, improve performance, and relax themanufacturing tolerances. In addition, similar to the design of FIG. 50,a second glass material, calcium fluoride, is used to increase thecorrection bandwidth. The design of FIG. 53 is corrected from 193 to 203nm. The FIG. 53 design has one calcium fluoride element in the eyepiecegroup and one calcium fluoride/silica doublet near the intermediateimage. The object extends from 0.25 mm to 0.75 mm off axis and thedesign has a bandwidth from 193.3–203.3 nm.

As shown in FIG. 53, light energy or laser energy is transmitted fromenergy source 53000 (not shown) and into objective 5300. As in objectiveaspects presented in FIG. 51 and FIG. 52, an external pupil plane 5301is available for aperturing and Fourier Filtering. Objective 5300includes first lens 5302 and first field lens arrangement 5330, whichincludes first field lens 5303 and second field lens 5304. This firstfield lens arrangement 5330 directs light energy to small folding mirroror reflecting surface 5305, which reflects energy toward focusing lenses5306 and 5307 and to Mangin mirror 5308. Light energy reflects back fromMangin mirror 5308 back through focusing lenses 5307 and 5306, and pastsmall folding mirror or reflecting surface 5305. Light energy thenpasses through second field lens arrangement 5331, which includes thirdfield lens 5309, fourth field lens 5310, and fifth field lens 5311.Light then passes through focusing lens arrangement 5332, which includesfirst focusing lens 5312, second focusing lens 5313, third focusing lens5314, fourth focusing lens 5315, fifth focusing lens 5316, sixthfocusing lens 5317, seventh focusing lens 5318. The specimen or surface5319 to be examined is not shown in FIG. 53, but is located to the rightof the objective 5300 in the orientation shown in FIG. 53. Light energystrikes the specimen 5317 and reflects back through the objective 5300of FIG. 53.

The surface data for the design of FIG. 53 is listed in Table 5.

TABLE 5 Surface data for a linear design as in FIG. 53 at 193 nm with a10 nm bandwidth Element Surf Radius Thickness Material Number OBJInfinity Infinity N/S STO Infinity 35.063 5301 2 527.763 4.000 Silica5302 3 −32.542 8.204 5302 4 572.044 2.000 Silica 5303 5 16.118 1.4475303 6 17.545 5.000 CaF2 5304 7 −65.290 35.838 5304 8 Infinity 0.000Mirror 5305 9 Infinity −96.873 5305 10 −795.110 −10.000 Silica 5306 1192.663 −199.957 5306 12 64.768 −8.000 Silica 5307 13 281.874 −14.0195307 14 38.871 −9.000 Silica 5308 15 80.841 9.000 Mirror 5308 16 38.87114.019 5308 17 281.874 8.000 Silica 5307 18 64.768 199.957 5307 1992.663 10.000 Silica 5306 20 −795.110 96.873 5306 21 Infinity 9.399 530622 −16.045 2.000 Silica 5309 23 24.034 0.217 5309 24 24.751 4.000 CaF25310 25 −16.515 22.166 5310 26 38.990 4.000 Silica 5311 27 41.231164.091 5311 28 1232.004 10.000 Silica 5312 29 −95.859 1.000 5312 3079.148 9.000 Silica 5313 31 −1367.718 33.541 5313 32 −59.580 4.000Silica 5314 33 201.391 45.680 5314 34 173.228 4.500 Silica 5315 3558.668 7.500 5315 36 −127.932 6.000 Silica 5316 37 −41.246 1.000 5316 3831.082 38.000 Silica 5317 39 153.068 1.000 5317 40 17.627 12.491 Silica5318 41 35.390 11.566 5318 IMA Infinity

A complete imaging system, such as presented in FIG. 57 and 46, requiresan objective, such as presented in FIGS. 47–53, and image formingoptics. The image forming optics can be many different designs. Theimage forming optics can be a static tube lens capable of producing asingle magnification. In this case, different magnifications areachieved by using different tube lenses. This type of image formingoptics can be designed by someone skilled in the art. Another type ofimage forming optics is a zooming tube lens. A zooming tube lens has theadvantage that only a single optical system is required to produce awide range of magnifications. Examples of two different types of zoomingtube lenses will be presented in design aspects 7 and 8. It is importantthat the image forming optics are corrected for the wavelength andspectral bandwidth of the illumination source.

The seventh aspect illustrated in FIG. 54 is an image forming tube lensthat uses a varifocal two motion zoom to change magnifications. Thedesign methodology is similar to that presented above. The designcomprises a stationary doublet 5420, a zooming group 5421, and adetector group 5422. The stationary doublet consists of a first doubletlens 5402 and a second doublet lens 5403. The zooming group consists ofa first zoom lens 5404, a second zoom lens 5405, and a third zoom lens5406. The detector group 5422 consists of a protective window 5407 and adetector 5408. Different magnifications are achieved by moving the zoomgroup along the optical axis and then moving the detector with theprotective window to refocus. Three example magnifications are shown inFIG. 55. The low magnification zoom position 5501 has the shortest totallength with the zoom group 5521 farthest away from the doublet 5520. Forthe medium magnification 5502 the total length from the doublet 5520 tothe detector group 5522 increases and the distance from the doublet 5520to the zoom group 5521 decreases. For the high magnification 5503 themaximum distance from the doublet 5520 to the detector group 5522 isachieved and the distance from the doublet 5520 to the zoom group 5521is minimized. This design is capable of magnifications from 38 times togreater than 152 times. Over the magnification range from 38 times to152 times, the total length of the system from the doublet 5520 to thedetector 5508 changes from 320 mm to 880 mm. The three lenses that movein the zooming group 5521 move by 68 mm. The pupil 5501 of the design inFIG. 55 is matched to the design presented in FIG. 51. It is possiblefor someone skilled in the art, when presented with this disclosure, todesign a similar tube lens for use with the objective designs presentedin FIGS. 47–53 as well as other designs. The design of a varifocal twomotion zoom with other magnifications and magnification ranges is alsopossible. The surface data for the design of FIG. 54 is listed in Table6.

TABLE 6 Surface data for an image forming tube lens as in FIG. 54 at 193nm with a 1.5 nm bandwidth Surface Surf Radius Thickness Glass numberOBJ Infinity Infinity N/S STO Infinity 20.000 5401 2 121.600 3.000 Caf25402 3 −77.915 0.500 5402 4 −78.860 2.000 Silica 5403 5 −1820.981 90.7325403 6 106.439 2.500 Caf2 5404 7 59.162 99.528 5404 8 50.266 4.000Silica 5405 9 124.701 14.006 5405 10 −687.986 3.000 Caf2 5406 11 53.069470.445 5406 12 Infinity 1.000 Silica 5407 13 Infinity 1.000 5407 IMAInfinity 5408

The eighth aspect of the DUV/VUV design illustrated in FIG. 56 is animage forming tube lens that uses an optically compensated single motionzoom to change magnifications. The design methodology is similar to thatin the paper by David R. Shafer, “Catadioptric optically compensatedzooming system with one moving element” Proc SPIE Vol. 2539, p235–240.The design consists of a first doublet 5620, a zooming group 5621, amirror 5606, a second doublet 5622, and a detector 5610. The firstdoublet consists of a doublet lens 5602 and a following doublet lens5603. The zooming group consists of a first zoom lens 5604 and a secondzoom lens 5605. Different magnifications are achieved by moving the zoomgroup 5621 along the optical axis of the first doublet 5620, the zoomgroup 5621, and the mirror 5606. No other motion is required. Threeexample magnifications are shown in FIG. 57. The low magnification zoomposition 5701 has the zoom group 5721 very close to the first doublet5720. For the medium magnification 5702, the zoom group is in betweenthe first doublet 5720 and the mirror 5704. For the high magnification5703 the zoom group 5721 is very close to the mirror 5704. The design iscapable of magnifications from 60 times to greater than 180 times. Thepupil 5601 of the design in FIG. 56 is matched to The design presentedin FIG. 51. It is possible for someone skilled in the art, whenpresented with this disclosure, to design a similar tube lens for usewith the objective designs presented in FIGS. 47–53 as well as otherdesigns. It is also possible for someone skilled in the art, whenpresented with this disclosure, to design an optically compensatedsingle motion zoom with other magnifications and magnification ranges.The surface data for the design of FIG. 56 is listed in Table 7.

TABLE 7 Surface data for an image forming tube lens as in FIG. 56 at 193nm with a 1.5 nm bandwidth Surface Surf Radius Thickness Glass numberOBJ Infinity Infinity N/S STO Infinity 20.000 5601 2 — 0.000 — N/S 3123.897 8.000 caf2 5602 4 −213.430 33.518 5602 5 −119.986 4.000 silica5603 6 206.676 315.675 5603 7 24267.100 4.000 caf2 5604 8 117.959 10.5775604 9 125.844 5.000 silica 5605 10 325.138 319.238 5605 11 −1125.668−319.238 MIRROR 5606 12 325.138 −5.000 silica 5605 13 125.844 −10.5775605 14 117.959 −4.000 caf2 5604 15 24267.100 −305.678 5604 16 — 0.000 —N/S 17 Infinity 0.000 MIRROR 5607 18 — 0.000 — N/S 19 Infinity 36.000N/S 20 39.403 6.000 silica 5608 21 81.584 0.500 5608 22 50.614 4.000silica 5609 23 33.375 11.140 5609 IMA Infinity 5610

Autofocus

As the semiconductor device moves during high speed inspection, minutechanges in the focus position must be corrected. Thus such a system maybe served by employing automatic focusing to maintain a high fidelityimage.

The present system may employ an autofocus subsystem in connection withthe positioning subsystem to automatically focus the light energyreceived from the illumination subsystem. Many different types ofautomatic focusing subsystems have been successfully applied tosemiconductor inspection. These automatic focusing subsystems consist ofdetecting a focus change, focusing the wafer or photomask, and usingfeedback to maintain the desired focus position.

Various techniques exist for detecting a focus change. One such methoddescribed in U.S. Pat. No. 4,639,587, assigned to KLA Instruments, theassignee of the present invention and hereby incorporated by reference,describes an automatic focusing system that uses the comparison of twomasks and is used primarily for semiconductor wafer inspection. Thistechnique has the advantage that it can be used to measure the focusposition of a wafer containing a partially fabricated integratedcircuit. Measuring the best focus position on a wafer containing apartially fabricated integrated circuit is complicated by the fact theremay be multiple layers with complex geometries with varyingreflectivities. The desired focus position is usually the top most layerof the wafer. However separating an actual focus change from a change inthe circuit patterns can be a difficult task. This technique produces abest focus location that is an average of the different levels on thewafer multiplied by the reflected signal. Focus location and correctionis less of an issue at short wavelengths where materials may be morestrongly absorbing.

Another method of automatic focus involves astigmatic focusing on aquadrant detector. The astigmatic focusing technique is commonly used inphotomask inspection and in Compact Disc readers and writers. In thismethod, light from an illumination source is focused on the samplesurface, typically through the imaging subsystem optics. The reflectedlight, typically collected by the imaging subsystem optics, is thenfocused by an astigmatic lens onto a quadrant detector. As the sample ismoved through focus, the shape of the focus changes and is measured bythe quadrant detector. This astigmatic focusing technique typicallyworks effectively for samples with limited topology variations.

The pulsed nature of the excimer laser can complicate the automaticfocusing method if pulsed light from the excimer laser is used for thefocusing. This may allow the focus position of the sample to be measuredduring each pulse, which may be adequate on a sample with minimaltopology variations or very high precision stage. To address this issue,one option is to use a continuous or nearly continuous energy source tomaintain focus when the specimen is not illuminated by pulsed light.

The method for detecting the focus change must also account forseparation of the automatic focusing signal from the image in theimaging subsystem. Both variable wavelength and different field aspectsmay be incorporated. If a different wavelength is used between theautofocus and the imaging subsystems, a dichroic device such as abeamsplitter or grating may be used to separate the signals. If aslightly different position on the sample is used for the autofocus andimaging subsystems, the signals can be separated at an internal fieldplane within the imaging subsystem or at the final image plane. In thiscase, the same illumination source can be used for the illumination andautofocus subsystems.

Several methods can be used to focus the semiconductor device. Movingthe semiconductor device itself to maintain focus is typically doneduring wafer inspection. Moving the objective to maintain focus istypically performed for photomask inspection. For transmitted light, thecondenser focusing the light on the photomask may also be moved. Forlarge high precision optical systems such as previously described, itmay not be feasible to move the objective for focusing. In thissituation, one or more of the optical elements in the imaging subsystemmay be used to compensate for focus changes. Preferable performance mayresult when focusing does not greatly affect the magnification ortelecentricity of the imaging subsystem.

Feedback control may be employed to maintain the proper focusing. Thefeedback control may take into account the resonance of the differentautofocus mechanical and electronic components and minimize overshootand ringing. Such feedback controls are used in autofocus systems forsemiconductor inspection, compact disc players, and other high precisionoptical devices. The particular feedback loop parameters, such as thoseused in a Proportional Integral Derivative (PID) loop controller, andare specific to the autofocus subsystem design.

Sensor

One type of sensor that may be employed in the present system is backthinned silicon. A back thinned silicon sensor has high speed with lownoise readout, high quantum efficiency, long lifetimes, and high MTF.Many other types of sensors may be employed, including but not limitedto front side devices with open silicon areas, lumogen coated front sidesensors, photo-diamond sensors, and silicon carbide sensors.Photo-diamond type and silicon carbide type sensors tend to have verylittle sensitivity to visible wavelengths.

These sensor types can be operated in different modes including frametransfer and time delay and integration (TDI). The frame transfer modeis useful for an inspection system using a single excimer laser pulse toilluminate an area on the sample. Each pulse generally corresponds toone frame of the sensor. This has the advantage that two halves of thedetector can be read out simultaneously for increased data rates. Ifmultiple pulses from the excimer laser are used to expose a single areaon the sample, a TDI mode sensor can be used. In special inspectionmodes, such as the confocal and dark field inspection modes, singlepoint detectors or arrays of single point detectors may also be used.

The system performs a high speed sample inspection with high resolution.For example an inspection system with a pixel size of 50 nm wouldrequire a data rate of 1.1 Gpixels/second to scan an area of 10 cm×10 cmin one hour. Increasing the inspection speed tends to reduce theper-sample cost of an inspection system. The sensor may also have verylow noise at these high data rates. For example less than 1 count ofnoise out of 256 counts of signal. Often less than 1 count of noise outof 1024 counts of signal is required. To obtain low noise is anextremely complex issue that involves careful design of the sensorlayout, amplifier, packaging, and readout electronics by one skilled inthe art. The electrical design of each of these is critical to minimizethe effects of crosstalk, feedthrough and adequately isolate the ground.The sensor subsystem also has high quantum efficiency, long lifetimes,and a high contrast transfer function. High quantum efficienciesgenerally require less light from the illumination system to fullyexpose the sensor. In this scenario, a smaller excimer laser can be usedfor the illumination subsystem. A smaller laser can have longerlifetimes. Higher quantum efficiency also means less energy is requiredon the sample surface, thus tending to limit the potential for damagefrom the high peak powers of an excimer laser pulse. Long lifetimesminimize the possibility that the sensor performance changes with time,decreasing the risk of system recalibration. Typical performance changeswith excimer laser exposure are an increase in dark current and adecrease in quantum efficiency. If these changes are too large, thesensor may be out of range for recalibration and have to be replaced. Ahigh Contrast Transfer Function (CTF) is required to detect the imagewith adequate resolution. If the imaging subsystem produces a very highresolution image, the inspection system will not be able to detect thehigh resolution image if the sensor has a low CTF. A CTF generally aslow as approximately 0.4 is acceptable for an inspection system, howevera value of 0.6 or greater may be employed with acceptable results incertain conditions.

The sensor employed in the present system may be a single point diodetype detector or an area type detector such as a CCD or a CCD operatingin the Time Delay and Integration (TDI) mode. This sensor may have ahigh quantum efficiency, low noise, and a good Modulation TransferFunction (MTF). Back thinned CCD sensors can be used for this purpose.

One possible sensor that may be employed in the current design ispresented in U.S. Pat. No. 4,877,326, entitled “Method and Apparatus forOptical Inspection of Substrates,” assigned to the assignee of thepresent application, the entirety of which is incorporated herein byreference.

The sensor may be back illuminated or front illuminated, where frontillumination may include virtual phase design, solid state, with openareas to be UV sensitive, and may incorporate sensors with florescentcoatings. The system may be a point, line, 2D, multitap readout, linear,photodiode array, CCD, or split readout to double the speed. The sensormay be a diamond based sensor, and may have antiblooming capability. Thesensor may be staggered or comprise multiple sensors in one package.Sensor electronics may provide for exposure correction. Adjacent imagesor total power may be viewed by the sensor and sensor readings may beused to correct long term drift in the laser or correct jitter.

The sensor employed may include aspects of high quantum efficiency atthe excimer laser wavelength. Back thinned silicon sensors may beemployed to offer adequate performance. The sensor further may have highresolution capabilities to support high resolution imaging, high speedcapability to support high speed inspection, and low noise and highdynamic range to support the various defect detection modes contemplatedherein.

Data Acquisition

The data acquisition subsystem for the current system includes framemode operation and TDI mode operation. When operating in frame mode,only a single laser pulse exposes a frame as the positioning stagescans. In this mode, the effects of stage vibration are reduced byvirtue of the short exposure pulse, and improved sensor MTF over asensor, such as a TDI sensor. TDI mode entails integrating multipleexcimer laser pulses. This helps improve speckle smoothing and reducepeak powers as described in the illumination subsystem.

The data acquisition subsystem can use a single sensor, which may have alarge area for sensing in accordance with the description above. Thesensor may fill the imaging field of view to maximize the available areaand decrease peak powers.

Multiple sensors may also be employed to reduce the overall cost of thesensor, as use of more small sensors is typically less than use of onelarge sensor of similar area. These sensors may be located in relativelyclose proximity. They can be mounted on the same electronics board andeven butted together effectively producing a larger sensor. The sensorscan also be spatially separated from each other. This can have anadvantage because it may be difficult to pack all the readoutelectronics near the location of the sensor. The field if the imagingsubsystem can be split into multiple parts using a scraping mirror, beamsplitter, prism, grating, or diffractive optic. Each part can then besent to one sensor. Ideally, the splitting is done at a field plane sothe impact on the image fidelity is minimized. It is also possible tolocate the sensors at different focal positions to gather in and out offocus data simultaneously. It is also possible to have different imagingmodes on the different sensors to simultaneously gather defect data. Forexample, bright field and dark field data could be gatheredsimultaneously to look for different types of defects.

Additional difficulties arise using high speed sensors in the presenceof an excimer laser. The sensor readout and excimer laser may besynchronized, usually with the sample being inspected.

Synchronization entails matching the timing of the excimer laserpulsing, the sensor readout, and the desired location of the positioningstage to each other. Typically, one of these devices is used as areference and the others are synchronized to it. One such method uses asensor and laser that are synchronized to the positioning subsystem.Synchronization of the sensor readout, positioning stage, and laseroccur by the stage producing a timing signal.

An additional method may use a sensor and positioning subsystemsynchronized to the laser. A further method would allow the laser andpositioning subsystem to be synchronized to the sensor.

Data acquisition can occur as the stage accelerates and decelerates, butthe synchronzation between the elements provides the improvedcharacteristics of data acquisition for the design.

For an excimer laser, the first few pulses received after the laser hasnot been pulsing may be unstable and can be discarded. This will reducethe dynamic range over which the exposure must be corrected. The systemmay also include selectable frequency, pulse skipping, and adjustablepower. To optimize the sensor exposure during the scanning of thepositioning stage.

Data Analysis

The purpose of the data analysis subsystem is to identify yield limitingdefects on a sample. Defects are primarily identified using comparisontechniques. One comparison technique used for wafer and photomaskinspection uses the comparison of different dies. For example, if dies 1and 2 are compared and a difference is found at location A and dies 2and 3 are compared and a difference is also found at location A, thedefect at location a is attributed to die 2.

Another technique uses a comparison between different cells within adie. A cell is defined such that it repeats many times within theinspection area of interest. This type of comparison is useful formemory, and logic areas within a die. It is often desired to have anadjustable magnification in the imaging subsystem to each cell can beadjusted to an integral number of sensor pixels.

A third comparison technique is die-to-database comparison. This isuseful for inspecting photomasks because they are relatively simplestructures and their desired patterns are precisely known in anelectronic format or database. In order to compare data from theinspection system to the database, the database must be rendered bytaking into account the performance of the imaging and sensor subsystemsand their effect on the database. This rendered database can then becompared to the data gathered by the inspection system. It is alsopossible to take inspection data and render it to the database forcomparison, however this can be more difficult. It has its mainadvantage for the aerial imaging inspection mode. Aerial imaging iscomplicated by the fact that the errors in the mask must be inferredfrom the measured data. Compensation may also be done for fieldaberrations like distortion.

The data is acquired in a frame-by-frame basis. Each frame can come froma single excimer laser pulse or multiple excimer laser pulses. Eachframe can be allowed to slightly overlap with the previous andsubsequent frames so no data is lost. This overlapping region can alsobe used for accurately aligning the frames.

Alignment of the comparison data is a major challenge for data analysis.It is often desirable for the data being compared to contain an integernumber of pixels. It can also be desirable for some frames of data tobegin at known locations to simplify comparison. For example, indie-to-die comparison, it simplifies the computation if the beginning ofeach die is in the same location within a frame. This can beaccomplished by accurately adjusting the timing of the acquisitionsystem and adjusting the frame overlap a desired amount so the beginningon a first die and the beginning of a second die are located at the sameposition within a frame.

It may also be desirable to have an auxiliary light source that can beused for frame alignment. This light source can be continuous and theframe alignment and timing checked prior to the arrival of the excimerpulse. Any variation in the desired position can be compensated for byadjusting the timing of the excimer laser pulse.

The defect data can then be sent to other systems for further analysissuch as e-beam review, macro review, or focused ion beam destructiveanalysis. The data can also be sent to yield management software for usein fab wide yield improvement.

While the invention has been described in connection with specificembodiments thereof, it will be understood that the invention is capableof further modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as come within known and customary practice withinthe art to which the invention pertains.

1. A system for inspecting a specimen comprising: an illuminationsubsystem comprising a relatively low coherence excimer laser fordirecting low coherence light energy toward said specimen wherein lightenergy originating from said excimer laser is generated using aplurality of gasses within the excimer laser and light energyoriginating from said excimer laser is provided to said specimen using anumerical aperture in excess of approximately 0.90; and a sensorsubsystem for receiving light energy reflected from said specimen;wherein said sensor subsystem comprises a silicon sensor and saidspecimen comprises a semiconductor specimen.
 2. The system of claim 1where the silicon sensor comprises a back thinned sensor.
 3. The systemof claim 2 where the silicon sensor is operated in a predeterminedsensing mode.
 4. The system of claim 1 where the silicon sensor hasanti-blooming capability.
 5. A system for inspecting a specimencomprising: an illumination subsystem comprising a relatively lowcoherence excimer laser for directing low coherence light energy towardsaid specimen, said excimer laser generating the light energy from aplurality of gasses within a cavity of said excimer laser and saidcavity comprising at least one dispersive component configured to narrowtransmittal bandwidths of the light energy; and a sensor subsystem forreceiving light energy reflected from said specimen.
 6. A system forinspecting a specimen comprising: an illumination subsystem comprising alow coherence-excimer laser for directing low coherence light energytoward said specimen, said excimer laser comprising a cavity comprisingat least one dispersive component configured to narrow transmittalbandwidths of the light energy and at least one curved reflectivesurface configured to reduce spatial coherence of the light energy; anda sensor subsystem for receiving light energy reflected from saidspecimen.
 7. A system for inspecting a specimen comprising: anillumination subsystem comprising a reduced coherence excimer lasercomprising a cavity comprising at least one curved reflective surfaceconfigured to reduce spatial coherence of the excimer laser, saidexcimer laser configured to direct low coherence light energy towardsaid specimen using a numerical aperture in excess of approximately0.65; a sensor subsystem comprising a detector for detecting reflectedlight energy received from said specimen; and a data analysis subsystemreceiving image data from said sensor and using image comparisons toidentify defects on said specimen.
 8. The system of claim 7 where thecomparison is a die-to-die comparison.
 9. The system of claim 8 whereany frame overlap is adjusted in the data analysis subsystem to align aportion of one die with a particular pixel in the sensor subsystem. 10.The system of claim 7 where the comparison system is a cell-to-cellcomparison.
 11. The system of claim 7 where the comparison is adie-to-database comparison.
 12. The system of claim 11 where thedatabase is rendered to an image for comparison.
 13. The system of claim11 where the image is rendered to a database for comparison.
 14. Asystem for inspecting a specimen comprising: an illumination subsystemcomprising a relatively low coherence excimer laser transmitting lowcoherence light energy toward said specimen, said excimer lasercomprising a cavity employing at least one dispersive componentconfigured to narrow transmission bandwidth of the light energytransmitted toward the specimen; a sensing subsystem for sensing lightenergy received from said specimen, said sensor subsystem producing asensor readout; and a data acquisition subsystem that synchronizes theexcimer laser to the sensor readout.
 15. The system of claim 14, furthercomprising an imaging subsystem between said illuminating subsystem andsaid specimen, wherein the data acquisition subsystem employs at leastone sensor within the imaging subsystem field of view.
 16. The system ofclaim 14, further comprising an imaging subsystem between saidilluminating subsystem and said specimen, wherein the data acquisitionsubsystem that employs a plurality of sensors within the imagingsubsystem field of view.
 17. A system for inspecting a specimencomprising: an illumination subsystem comprising a relatively lowcoherence excimer laser and at least one catadioptric element, saidillumination subsystem employed to transmit low coherence light energytoward the specimen, said excimer laser comprising a cavity employing atleast one dispersive component configured to narrow transmissionbandwidth of the light energy transmitted toward the specimen; apositioning subsystem for positioning the specimen in a desiredorientation, the positioning subsystem comprising a positioning stage; asensing subsystem for sensing low coherence light energy received fromsaid specimen, said sensor subsystem producing a sensor readout; and adata acquisition subsystem that synchronizes the sensor readout to thepositioning stage.
 18. The system of claim 17, further comprising animaging subsystem between said illuminating subsystem and said specimen,wherein the data acquisition subsystem employs at least one sensorwithin the imaging subsystem field of view.
 19. The system of claim 17,further comprising an imaging subsystem between said illuminatingsubsystem and said specimen, wherein the data acquisition subsystem thatemploys a plurality of sensors within the imaging subsystem field ofview.
 20. A system for inspecting a specimen comprising: an illuminationsubsystem comprising a relatively low coherence excimer laser configuredto direct low coherence light energy toward the specimen; a sensingsubsystem for sensing light energy received from said specimen, saidsensor subsystem producing a sensor readout; and a data acquisitionsubsystem that synchronizes the sensor readout to the excimer laser. 21.The system of claim 20, further comprising an imaging subsystem betweensaid illuminating subsystem and said specimen, wherein the dataacquisition subsystem employs at least one sensor within the imagingsubsystem field of view.
 22. The system of claim 20, further comprisingan imaging subsystem between said illuminating subsystem and saidspecimen, wherein the data acquisition subsystem that employs aplurality of sensors within the imaging subsystem field of view.
 23. Thesystem of claim 21 where each sensor is located proximate within a fieldof view of the imaging subsystem.
 24. The system of claim 23 whereineach sensor is physically spaced from any other sensor; and an imagingsubsystem field comprises a plurality of portions and each portion ofthe field is sent to a different sensor.
 25. A method of inspecting aspecimen, comprising: illuminating at least a portion of said specimenusing a relatively low coherence excimer source using at least onerelatively intense wavelength, said illuminating occurring at anumerical aperture in excess of approximately 0.97; detecting radiationreceived from said illuminated portion of said specimen; analyzing saiddetected radiation to view potential defects present in said portion ofsaid specimen; and exposing at least a portion of said specimen to asecond, relatively weak wavelength from said excimer source.
 26. Themethod of claim 25, wherein said relatively weak wavelength is used toassist in focusing said specimen to a desired location on said specimen.27. A method of inspecting a specimen, comprising: illuminating at leasta portion of said specimen using a relatively low coherence excimersource using at least one relatively intense wavelength from saidsource, said illuminating occurring at a numerical aperture in excess ofapproximately 0.97; detecting radiation received from said illuminatedportion of said specimen; analyzing said detected radiation to viewpotential defects present in said portion of said specimen; and exposingat least a portion of said specimen to a second, relatively weakwavelength without concurrently exposing said portion to said relativelyintense wavelength, wherein said relatively weak wavelength may be usedto assist in focusing said specimen.
 28. A method of inspecting aspecimen, comprising: illuminating at least a portion of said specimenusing a relatively low coherence excimer source; detecting radiationreceived from said illuminated portion of said specimen; analyzing saiddetected radiation to detect potential defects present in said portionof said specimen; and monitoring the dose of excimer radiation to whichsaid portion of said specimen is exposed; wherein illumination of saidportion is discontinued once a predetermined dose limit is reached. 29.The method of claim 28, wherein said illumination is terminatedautomatically upon reaching the predetermined dose limit.
 30. The methodof claim 28, wherein data concerning exposure dose as a function ofposition is maintained for said specimen.
 31. The method of claim 30,wherein said data is stored and used to limit overexposure of saidspecimen during subsequent processing steps.
 32. The method of claim 31,wherein said data is stored in an open format accessible by a pluralityof process tools.
 33. A method of inspecting a specimen, comprising:illuminating at least a portion of said specimen using a relatively lowcoherence excimer source using at least one relatively intensewavelength from said source, said illuminating occurring at a numericalaperture in excess of approximately 0.97; detecting radiation receivedfrom said illuminated portion of said specimen; and exposing at least aportion of said specimen to a second, relatively weak wavelength fromsaid excimer source.
 34. The method of claim 33, further comprisinganalyzing said detected radiation to view potential defects present insaid portion of said specimen subsequent to said detecting.
 35. Themethod of claim 33, wherein said relatively weak wavelength is used toassist in focusing a desired location on said specimen.
 36. A method ofinspecting a specimen, comprising: illuminating at least a portion ofsaid specimen using a relatively low coherence excimer source using atleast one relatively intense wavelength from said source; detectingradiation received from said illuminated portion of said specimen; andexposing at least a portion of said specimen to a second, relativelyweak wavelength without concurrently exposing said portion to saidrelatively intense wavelength, wherein exposing said portion to saidrelatively weak wavelength assists in navigating to a desired locationon said specimen.
 37. The method of claim 36, further comprising:analyzing said detected radiation to view potential defects present insaid portion of said specimen prior to said exposing.
 38. An apparatusfor inspecting a specimen, comprising: an illuminator configured toilluminate at least a portion of said specimen using a relatively lowcoherence excimer source; a detector configured to detect radiationreceived from said illuminated portion of said specimen; and a monitorconfigured to monitor a dose of excimer radiation to which said portionof said specimen is exposed; wherein said illuminator discontinuesillumination of said portion once a predetermined dose limit is reached.39. The apparatus of claim 38, further comprising means for analyzingsaid detected radiation to detect potential defects present in saidportion of said specimen subsequent to said detecting.
 40. The apparatusof claim 38, wherein said illuminator discontinues illumination based ondata received from said monitor.
 41. The apparatus of claim 38, whereinsaid illumination is terminated automatically upon reaching thepredetermined dose limit.
 42. The apparatus of claim 38, wherein dataconcerning exposure dose as a function of position is maintained forsaid specimen.
 43. The apparatus of claim 42, wherein said data isstored and used to limit overexposure of said specimen during subsequentprocessing.
 44. The apparatus of claim 43, wherein said data is storedin an open format accessible by a plurality of process tools.